Isolated power converter having a voltage supply circuit

An isolated power converter includes: a transformer having primary winding and first and second auxiliary windings on the primary side; a converter stage configured to convert a DC input for driving the primary winding and having a resonant capacitor electrically connected to the primary winding; a controller configured to control switching of the converter stage; and a voltage supply circuit configured to select a first voltage as a supply voltage for the controller if a voltage proportional to a secondary side voltage of the transformer is at a first level or select a second voltage as the supply voltage if the voltage proportional to the secondary side voltage is at a second level greater than the first level. The first voltage corresponds to a summation of voltages across the first auxiliary winding and the resonant capacitor. The second voltage corresponds to a voltage across the second auxiliary winding.

BACKGROUND

Power adapters are typically designed to charge or supply multiple types of electronic devices such as laptop computers, tablets, mobile devices, etc. This requires a wide output voltage range, e.g., 3.3V to 20V in the case of the USB-PD (USB power delivery) specification. Other electronic devices such as smart speakers, sound bars, etc. can be powered using a USB-PD type adapter and usually require a higher voltage, e.g., around 24V. In the case of USB-PD as the power supply, the output voltage range must be extended up to 24V. Similar wide output voltage power supply requirements can be found in LED (light emitting diode) lighting power supply applications where a variable LED load can be connected.

The input voltage for USB-PD power adapters is usually in the range of 90 Vac to 264 Vac, where the flyback derived topology is widely used because of its inherent wide operating input and output voltage range. Examples of flyback variants used in USB-PD power adapters include fixed frequency/QR (quasi-resonant) flyback and the more efficient ZVS (zero-voltage switching) variant such as the active clamp flyback (ACF) and asymmetrical half bridge flyback.

Regardless of the topology, an auxiliary voltage must be generated to bias the controller on the primary side of the power adapter. Auxiliary/self-supply is an important aspect of the adapter design, since the auxiliary/self-supply affects multiple aspects of power supply operation such as start-up, standby power, and efficiency. Average efficiency specifications based on different regulation (e.g., DOE6 and COC v5 Tier2) must be met for every output voltage. Full load efficiency for the highest output power also is important for power density and thermal requirements. In addition, efficiency requirements for light-load conditions (e.g., 1 W, 1.5 W, etc.) must be met for every output voltage.

Thus, there is a need for an improved self-supply method for power adapters having a wide input voltage range (e.g., 90 Vac to 264 Vac) and a wide output voltage range (e.g., 3.3V to 24V).

SUMMARY

According to an embodiment of an isolated power converter, the isolated power converter comprises: a transformer comprising a primary winding, a first auxiliary winding, and a second auxiliary winding each on a primary side of the transformer; a converter stage configured to convert a DC input for driving the primary winding of the transformer, the converter stage comprising a resonant capacitor electrically connected to the primary winding; a controller configured to control switching of the converter stage; and a voltage supply circuit configured to select a first voltage as a supply voltage for the controller if a voltage proportional to a secondary side voltage of the transformer is at a first level or select a second voltage as the supply voltage if the voltage proportional to the secondary side voltage is at a second level greater than the first level, wherein the first voltage corresponds to a summation of a voltage across the first auxiliary winding and a voltage across the resonant capacitor, wherein the second voltage corresponds to a voltage across the second auxiliary winding.

DETAILED DESCRIPTION

The embodiments described herein provide a self-supply method for power adapters having a wide input voltage range (e.g., 90 Vac to 264 Vac) and a wide output voltage range (e.g., 3.3V to 24V). For isolated power converters that use a transformer, the self-supply method is implemented using auxiliary windings on the primary side of the transformer and a resonant capacitor electrically connected to the primary winding. The self-supply method enables efficient power supply design for wide output voltage range and allows the use of cost-optimized components.

Described next, with reference to the figures, are exemplary embodiments of an isolated power converter that implements the self-supply.

FIG.1illustrates an embodiment of an (galvanically) isolated power converter100. The isolated power converter100includes a transformer102having a primary (‘pri’) winding P1, a first auxiliary winding P2, and a second (‘sec’) auxiliary winding P3each on the primary (input) side104of the transformer102. The transformer102of the isolated power converter100also has a secondary winding S1on the secondary (output) side106of the transformer102.

The isolated power converter100also includes a converter stage that converts a DC input ‘Vin’ to an AC voltage ‘Vpri’ for driving the primary winding P1of the transformer102. The converter stage108includes a resonant capacitor ‘C1’ electrically connected to the primary winding P1of the transformer102.

The isolated power converter100also includes a controller110to control switching of the converter stage108. For example, the controller110may generate a signal ‘CTRL’ that controls when the converter stage108drives the primary winding P1of the transformer102with the AC voltage Vpri and when the converter stage108does not drive the primary winding P1, e.g., to meet an output voltage regulation target.

The isolated power converter100also includes a voltage supply circuit112that provides a supply voltage A/cc′ to the controller110so that the controller110is ‘self-powered’. The voltage supply circuit112selects a first voltage ‘Vcc1’ as the supply voltage Vcc for the controller110if a voltage proportional to the secondary side voltage A/sec′ of the transformer102is at a first level. The voltage supply circuit112selects a second voltage ‘Vcc2’ as the supply voltage Vcc for the controller110if the voltage proportional to the secondary side voltage Vsec is at a second level greater than the first level. The first voltage Vcc1corresponds to a summation of a voltage ‘VP2’ across the first auxiliary winding P2of the transformer102and a voltage VC1′ across the resonant capacitor C1of the converter stage108. The second voltage Vcc2corresponds to a voltage ‘VP3’ across the second auxiliary winding P3of the transformer102. In one embodiment, the first voltage Vcc1is in a range of 3.3V to 9V and the second voltage Vcc2is in a range of 10V to 24V.

The isolated power converter100also may include an input rectifier and filter circuit114that generates the DC input voltage Vin to the converter stage108from AC mains. The isolated power converter100also may include an output rectifier and filter circuit116that generates a DC output voltage ‘Vout’ derived from the secondary side voltage Vsec of the transformer102. The input rectifier and filter circuit114and the output rectifier and filter circuit116may include a diode rectifier or a synchronous rectifier. For example, the DC input voltage Vin to the converter stage108may come from a full wave rectifier circuit used to convert the AC mains input, e.g., from 90V to 264 Vac into a rectified dc voltage Vin.

FIG.2illustrates an embodiment of the voltage supply circuit112of the isolated power converter100. According to this embodiment, the converter stage108is implemented as a resonant flyback stage having a high-side switch Q1such as a power MOSFET (metal-oxide semiconductor field-effect transistor), a low-side switch Q2such as a power MOSFET, and a resonant tank circuit. The high-side switch Q1and the low-side switch Q2are connected in series between the DC input Vin and ground ‘GND’ in a half bridge configuration.

The controller102drives the gate of the high-side switch Q1with a first signal ‘HS_GD’ and the gate of the low-side switch Q2with a second signal ‘LS_GD’. The controller102generates the gate signals HS_GD, LS_GD so that the high-side and low-side switches Q1, Q2are not turned on at the same time. The controller102adjusts the duty cycle of the gate signals HS_GD, LS_GD to maintain regulation of the output voltage Vout at capacitor C3on the secondary side of the isolated power converter100.

The resonant tank circuit of the voltage supply circuit112includes the resonant capacitor C1and an inductor L1. The inductor L1electrically connects the switching node Vpri between the high-side switch and the low-side switch of the half bridge to a first terminal200of the primary winding P1of the transformer102. The inductor L1may be embodied as an external inductor. In another example, the leakage inductance of the transformer102may be used as the resonant inductor L1. A first terminal202of the resonant capacitor C1is electrically connected to a second terminal204of the primary winding P1. A second terminal206of the resonant capacitor C1is grounded. The controller102has a sense terminal ‘CS’ electrically coupled to the grounded terminal206of the resonant capacitor C1.

The voltage supply circuit112includes a first smoothing capacitor C2, a second smoothing capacitor C3, a first diode D1, and a voltage supply selector201. The anode of the first diode D1is electrically connected to a first terminal208of the first auxiliary winding P2. The cathode of the first diode D1is electrically connected to an input of the voltage supply selector201.

The first terminal202of the resonant capacitor C1is electrically connected to both a second terminal210of the first auxiliary winding P2and the second terminal204of the primary winding P1. Both a first terminal212of the first smoothing capacitor C2and a first terminal214of the second smoothing capacitor C3are electrically connected to the cathode of the first diode D1. A second terminal216of the first smoothing capacitor C2is electrically connected to the second terminal210of the first auxiliary winding P2and the second terminal218of the second smoothing capacitor C3is grounded.

A first terminal220of the second auxiliary winding P3is electrically connected to an input of the voltage supply selector201through a diode D2of the voltage supply circuit112. A capacitor C4of the voltage supply circuit112is electrically connected between the cathode of this diode D2and a second terminal222of the second auxiliary winding P3.

The voltage supply circuit112provides the supply voltage Vcc to the controller110over a wide output voltage variation, e.g., from a minimum of 3.3V to a maximum of 24V, via a combination of the auxiliary primary windings P2, P3and the resonant capacitor C1. The voltage across the second auxiliary winding P3located on the primary side is in phase with the voltage across the secondary winding S1and provides energy while switch Q2is on, and produces the second voltage Vcc2which is equal to the output voltage Vout multiplied by the turn ratio between the secondary winding S1and the second auxiliary winding P3as given by:
Vcc2=Vout*(Ns/Np3)  (1)
where Ns is the number of turns for the secondary winding S1and Np3is the number of turns for the second auxiliary winding P3.

For higher output voltages, e.g., in the range of 10V to 24V, the voltage supply selector201selects the second voltage Vcc2as the supply voltage input Vcc to the controller110. The first voltage Vcc1decreases with the output voltage Vout and at some point, Vcc1becomes insufficient to adequately power the controller110. The voltage supply selector201selects the second voltage Vcc2as the supply voltage input Vcc to the controller110under these conditions, to ensure the controller110is adequately supplied for lower output voltages, e.g., in the range of 3.3V to 9V.

The first voltage Vcc1is generated by combining the voltage VP2across the first auxiliary winding P2and the voltage VC1across the resonant capacitor C1, using diodes D1and D2and smoothing capacitors C2and C3. Using just the voltage VC1across the resonant capacitor C1to supply the controller102over a wide output voltage range such as 3.3V to 24V would be insufficient because this voltage is proportional to the output Vout and the turn ratio between the first auxiliary winding P2and the secondary winding S1. Increasing the turn ratio may be viable alternative to increase the voltage VC1across the resonant capacitor C1, but this option is limited due to the minimum input voltage and voltage stress in the secondary side rectification device D3.

FIG.3illustrates another embodiment of the voltage supply circuit112of the isolated power converter100. The embodiment shown inFIG.3is similar to the embodiment shown inFIG.2. InFIG.3, the voltage supply circuit112also includes an additional diode D4having an anode electrically connected to the second terminal204of the primary winding P1and a cathode electrically connected to the second terminal210of the first auxiliary winding P2. Also, the first terminal214of the second smoothing capacitor C3is electrically connected to the second terminal210of the first auxiliary winding P2.

The voltage VC1on the resonant capacitor C1is coupled to the second smoothing capacitor C3through diode D4. Diode D4also blocks the auxiliary circuit current from going into the resonant capacitor C1so as not to effect resonant tank circuit operation. The first auxiliary winding P2located on the primary side is in phase with the secondary winding S1and provides energy while switch Q2is on, and produces a voltage VP2proportional to the output voltage Vout. The second terminal210of the first auxiliary winding P2is connected to the first (positive) terminal214of the second capacitor C3, in which its voltage is the same as the resonant capacitor C1, while the other terminal208of the first auxiliary winding P2is connected to the anode of diode D1. By connecting the first auxiliary winding P2in this way, the voltage from the resonant capacitor C1is utilized in addition to the voltage generated from the first auxiliary winding P2. This voltage Vcc1is the sum up across the smoothing capacitors C2and C3which is then selected by the voltage supply selector201to supply the controller102for lower output voltages, e.g., in the range of 3.3V to 9V.

The self-supply method shown in bothFIG.2andFIG.3for the controller102reduces the number of turns of the first auxiliary P2winding, since the first auxiliary P2winding is used to supplement the voltage on the resonant capacitor C1for lower output voltages, e.g., in the range of 3.3V to 9V. This allows the flyback converter to work over a very wide range, with the voltage supply selector201selecting the second voltage Vcc2as the controller supply voltage Vcc for higher output voltages, e.g., in the range of 10V to 24V and selecting the first voltage Vcc1as the controller supply voltage Vcc for lower output voltages, e.g., in the range of 3.3V to 9V.

For example, using the asymmetrical half bridge flyback converter shown inFIG.2orFIG.3with a universal ac mains input and an output voltage of 5V to 24V, the turn ratio Ns/Np2is less than 2.4, e.g.,22/10. This turn ratio produces about 11V across the resonant capacitor C1at the minimum output voltage which is not high enough to reliably supply the controller102if the controller102requires, e.g., a 12V supply voltage. The turn ratio could be increased to 2.4 or even slightly higher to reliably power the controller102at the minimum output voltage. However, at a maximum output voltage of 24V, the duty cycle of the controller102would saturate, especially if a smaller bulk capacitor is used. By adding the first auxiliary winding P2as shown inFIGS.2and3, only 4 turns are needed on P2to produce 13V (11V+2V) which is an adequate Vcc bias voltage for the controller102in this example.

The significantly smaller number of turns for the first auxiliary winding P2(e.g., 4 turns) significantly reduces the voltage rating of rectifier diode D1and the first smoothing capacitor C2. For the example given above, this results in a voltage stress of about 67V (264*1.414/(22/4)) for rectifier diode D1as compared to (264*1.414/(22/26))˜441V using conventional two winding solutions. Accordingly, the rectifier diode D1may have a rated voltage of at most 100V instead of 600V. The same advantage also applies for the first smoothing capacitor C2, because of the smaller number of turns for the first auxiliary winding P2and how the first smoothing capacitor C2is connected inFIGS.2and3. Accordingly, the first smoothing capacitor C2may have a rated voltage of less than 100V. For example, the first smoothing capacitor C2for the first auxiliary winding P2would only need to be rated for 25V instead of 100V for the example given above. This lower voltage rating advantage translates to a smaller package size and reduced cost.

FIG.4illustrates another embodiment of the voltage supply circuit112of the isolated power converter100. The embodiment shown inFIG.4is similar to the embodiment shown inFIG.3. InFIG.4, the second terminal206of the resonant capacitor C1is electrically connected to ground by a current sense resistor R1and the second terminal218of the second smoothing capacitor C3is electrically connected to a node ‘CS’ between the current sense resistor R1and the second terminal206of the resonant capacitor C1. In this configuration, the resonant capacitor C1is connected to the current sense resistor R1instead of directly connected to ground and the second smoothing capacitor C3is connected on top of the current sense resistor R1at the CS node.

For current mode control, the current sense resistor R1may be used to detect the peak current on the primary winding P1. The voltage across the current sense resistor R1reflects the current in the primary winding P1and is relatively low (e.g., about 200 mV) and therefore does not adversely impact the much higher controller supply voltage Vcc when the voltage supply selector201selects the first voltage Vcc1as the controller supply voltage Vcc.

FIG.5illustrates another embodiment of the voltage supply circuit112of the isolated power converter100. The embodiment shown inFIG.5is similar to the embodiment shown inFIG.4. InFIG.5, the second terminal218of the second smoothing capacitor C3is directly connected to ground.

FIG.6illustrates another embodiment of the voltage supply circuit112of the isolated power converter100. The embodiment shown inFIG.6is similar to the embodiment shown inFIG.3. InFIG.6, the voltage supply selector201includes a depletion mode transistor Q3and a Zener diode Z1. The depletion mode transistor Q3has a drain D Q3electrically connected to a node300at the first voltage Vcc1and a source S Q3electrically connected to the power supply input VCC of the controller102. The Zener diode Z1has a cathode electrically connected to the gate G Q3of the depletion mode transistor Q3and an anode electrically connected to the second terminal218of the second smoothing capacitor C3. A resistor R2connects the gate G Q3of the depletion mode transistor Q3to the source S Q3of the depletion mode transistor Q3.

The Zener diode Z1turns off the depletion mode transistor Q3if the voltage proportional to the secondary side voltage Vsec is at the second level. A node302at the second voltage Vcc2is electrically connected to the power supply input VCC of the controller102, such that the supply voltage Vcc for the controller102is determined by the first voltage Vcc1if the depletion mode transistor Q3is on and determined by the second voltage Vcc2if the depletion mode transistor Q3is off. In one embodiment, the depletion mode transistor Q3is a normally-on GaN transistor. The second terminal206of the resonant capacitor C1may be directly connected to ground as shown inFIG.6, or instead may be electrically connected to ground by a current sense resistor R1, e.g., as shown inFIGS.4and5. The second terminal218of the second smoothing capacitor C3may be directly connected to ground as shown inFIG.6, or instead may be electrically connected to a node CS between the current sense resistor R1and the second terminal206of the resonant capacitor C1, e.g., as shown inFIG.5.

The depletion mode transistor Q3, resistor R2, and Zener diode Z1collectively function as the voltage supply selector201for the controller102, and as a linear regulator to limit the voltage of the first voltage Vcc1. For higher output voltages, bias comes from the second voltage Vcc2, while the first voltage Vcc1is not connected to the controller102because the depletion mode transistor Q3is off. The Vcc2biasing of the controller102occurs when the second voltage Vcc2is sufficiently higher than the diode voltage of the Zener diode Z1to produce a sufficient source-to-gate voltage that turns off the depletion mode transistor Q3, thereby disconnecting the first voltage Vcc1from the controller102. This operation is similar to the operation that occurs inFIGS.2and3, such that when the output voltage Vout decreases, e.g., to 9V or lower, the second voltage Vcc2also decreases and is no longer high enough to keep the depletion mode transistor Q3in the off state. Under these conditions, the depletion mode transistor Q3turns on and the first voltage Vcc1takes over as the supply voltage Vcc for the controller102.

The voltage supply circuit112embodiment illustrated inFIG.6may be used in any ofFIGS.1through5. More generally, the isolated power converter100may be implemented as another type of isolated power converter such as a forward converter, push-pull converter, half bridge converter, full bridge converter, single-ended primary-inductor converter (SEPIC), Cuk converter, etc. instead of a flyback converter. The embodiments illustrated inFIGS.2through6in the context of a flyback converter may be readily adapted to forward converters, push-pull converters, half bridge converters, full bridge converters, SEPICs, Cuk converters, etc. without departing from the spirit of the invention.

An isolated power converter, comprising: a transformer comprising a primary winding, a first auxiliary winding, and a second auxiliary winding each on a primary side of the transformer; a converter stage configured to convert a DC input for driving the primary winding of the transformer, the converter stage comprising a resonant capacitor electrically connected to the primary winding; a controller configured to control switching of the converter stage; and a voltage supply circuit configured to select a first voltage as a supply voltage for the controller if a voltage proportional to a secondary side voltage of the transformer is at a first level or select a second voltage as the supply voltage if the voltage proportional to the secondary side voltage is at a second level greater than the first level, wherein the first voltage corresponds to a summation of a voltage across the first auxiliary winding and a voltage across the resonant capacitor, wherein the second voltage corresponds to a voltage across the second auxiliary winding.

The isolated power converter of example 1, wherein the converter stage is a flyback stage that comprises a high-side switch, a low-side switch, and a resonant tank circuit, wherein the resonant tank comprises the resonant capacitor and an inductor, wherein the high-side switch and the low-side switch are connected in series between the DC input and ground, wherein a switching node between the high-side switch and the low-side switch is electrically connected to a first terminal of the primary winding by the inductor, and wherein a second terminal of the primary winding is electrically connected to the resonant capacitor.

The isolated power converter of example 2, wherein the voltage supply circuit comprises: a first smoothing capacitor; a second smoothing capacitor; a first diode; and a voltage supply selector, wherein an anode of the first diode is electrically connected to a first terminal of the first auxiliary winding and a cathode of the first diode is electrically connected to the voltage supply selector, wherein a first terminal of the resonant capacitor is electrically connected to both a second terminal of the first auxiliary winding and the second terminal of the primary winding and a second terminal of the resonant capacitor is electrically connected to ground, wherein both a first terminal of the first smoothing capacitor and a first terminal of the second smoothing capacitor are electrically connected to the cathode of the first diode, wherein a second terminal of the first smoothing capacitor is electrically connected to the second terminal of the first auxiliary winding.

The isolated power converter of example 3, wherein the second terminal of the resonant capacitor is electrically connected to ground by a current sense resistor, and wherein a second terminal of the second smoothing capacitor is electrically connected to a node between the current sense resistor and the second terminal of the resonant capacitor.

The isolated power converter of example 3, wherein the second terminal of the resonant capacitor is electrically connected to ground by a current sense resistor, and wherein a second terminal of the second smoothing capacitor is directly connected to ground.

The isolated power converter of any of examples 3 through 5, wherein the voltage supply selector comprises: a depletion mode transistor having a drain electrically connected to a node at the first voltage and a source electrically connected to a power supply input of the controller; and a Zener diode having a cathode electrically connected to a gate of the depletion mode transistor and an anode electrically connected to a second terminal of the second smoothing capacitor, wherein the Zener diode is configured to turn off the depletion mode transistor if the voltage proportional to the secondary side voltage is at the second level, wherein a node at the second voltage is electrically connected to the power supply input of the controller, such that the supply voltage for the controller is determined by the first voltage if the depletion mode transistor is on and determined by the second voltage if the depletion mode transistor is off.

The isolated power converter of example 6, wherein the depletion mode transistor is a normally-on GaN transistor.

The isolated power converter of any of examples 2 through 7, wherein the voltage supply circuit comprises: a first smoothing capacitor; a second smoothing capacitor; a first diode; a second diode; and a voltage supply selector, wherein an anode of the first diode is electrically connected to a first terminal of the first auxiliary winding and a cathode of the first diode is electrically connected to the voltage supply selector, wherein a first terminal of the resonant capacitor is electrically connected to the second terminal of the primary winding and a second terminal of the resonant capacitor is electrically connected to ground, wherein an anode of the second diode is electrically connected to the second terminal of the primary winding and a cathode of the second diode is electrically connected to a second terminal of the first auxiliary winding, wherein a first terminal of the first smoothing capacitor is electrically connected to the cathode of the first diode and a second terminal of the first smoothing capacitor is electrically connected to the second terminal of the first auxiliary winding, wherein a first terminal of the second smoothing capacitor is electrically connected to the second terminal of the first auxiliary winding.

The isolated power converter of example 8, wherein the second terminal of the resonant capacitor is electrically connected to ground by a current sense resistor, and wherein a second terminal of the second smoothing capacitor is electrically connected to a node between the current sense resistor and the second terminal of the resonant capacitor.

The isolated power converter of example 8, wherein the second terminal of the resonant capacitor is electrically connected to ground by a current sense resistor, and wherein a second terminal of the second smoothing capacitor is directly connected to ground.

The isolated power converter of any of examples 8 through 10, wherein the voltage supply selector comprises: a depletion mode transistor having a drain electrically connected to a node at the first voltage and a source electrically connected to a power supply input of the controller; and a Zener diode having a cathode electrically connected to a gate of the depletion mode transistor and an anode electrically connected to a second terminal of the second smoothing capacitor, wherein the Zener diode is configured to turn off the depletion mode transistor if the voltage proportional to the secondary side voltage is at the second level, wherein a node at the second voltage is electrically connected to the power supply input of the controller, such that the supply voltage for the controller is determined by the first voltage if the depletion mode transistor is on and determined by the second voltage if the depletion mode transistor is off.

The isolated power converter of example 11, wherein the depletion mode transistor is a normally-on GaN transistor.

The isolated power converter of any of examples 2 through 7, wherein the voltage supply circuit comprises: a depletion mode transistor having a drain electrically connected to a node at the first voltage and a source electrically connected to a power supply input of the controller; and a Zener diode configured to turn off the depletion mode transistor if the voltage proportional to the secondary side voltage is at the second level, wherein a node at the second voltage is electrically connected to the power supply input of the controller, such that the supply voltage for the controller is determined by the first voltage if the depletion mode transistor is on and determined by the second voltage if the depletion mode transistor is off.

The isolated power converter of example 13, wherein the depletion mode transistor is a normally-on GaN transistor.

The isolated power converter of any of examples 2 through 7, wherein the voltage supply circuit comprises: a first smoothing capacitor; a first diode; and a voltage supply selector, wherein an anode of the first diode is electrically connected to a first terminal of the first auxiliary winding and a cathode of the first diode is electrically connected to the voltage supply selector, wherein a first terminal of the resonant capacitor is electrically connected to the second terminal of the primary winding and a second terminal of the resonant capacitor is electrically connected to ground, wherein a first terminal of the first smoothing capacitor is electrically connected to the cathode of the first diode, wherein a second terminal of the first smoothing capacitor is electrically connected to a second terminal of the first auxiliary winding.

The isolated power converter of example 15, wherein the voltage supply circuit comprises: a depletion mode transistor having a drain electrically connected to a node at the first voltage and a source electrically connected to a power supply input of the controller; and a Zener diode electrically connected to a gate of the depletion mode transistor and configured to turn off the depletion mode transistor if the voltage proportional to the secondary side voltage is at the second level, wherein a node at the second voltage is electrically connected to the power supply input of the controller, such that the supply voltage for the controller is determined by the first voltage if the depletion mode transistor is on and determined by the second voltage if the depletion mode transistor is off.

The isolated power converter of example 16, wherein the depletion mode transistor is a normally-on GaN transistor.

The isolated power converter of any of examples 15 through 17, wherein an output voltage of the isolated power converter ranges from a minimum of 3.3V to a maximum of 24V, and wherein a turn ratio for the first auxiliary winding is less than 2.4.

The isolated power converter of any of examples 15 through 18, wherein an output voltage of the isolated power converter ranges from a minimum of 3.3V to a maximum of 24V, wherein the first diode has a rated voltage of at most 100V, and wherein the first smoothing capacitor has a rated voltage of less than 100V.

The isolated power converter of any of examples 1 through 19, wherein the first level is in a range of 3.3V to 9V, and wherein the second level is in a range of 10V to 24V.