Soft switched secondary side post regulator for DC to DC converter

A high frequency, full bridge, resonant DC to DC converter provides a main DC output voltage that is regulated by adjusting the phase shift of the input power to a main input transformer and at least one additional DC output voltage that is regulated on the secondary side of the power input transformer. A main DC output voltage is regulated by a full bridge resonant switching converter with lossless switching of input power devices. At least one additional secondary winding on the transformer supplies a second DC output voltage that is regulated independently of the main DC output voltage. Two switching devices are used for each auxiliary DC output to regulate each auxiliary output voltage by adjusting the length of the on time of the pulses from the transformer's auxiliary secondary windings. Soft switching techniques are used to ensure that the switching devices turn on when the voltage across them is effectively zero.

TECHNICAL FIELD

The invention relates generally to DC-to-DC converters and more particularly, to regulation of a DC output voltage on the secondary side of a power input transformer.

BACKGROUND

DC-to-DC converters are known for converting a first DC voltage to a second, regulated DC voltage. Typically, a first DC voltage is converted to a series of AC pulses that are then rectified and regulated to create a second DC voltage. In many instances the AC pulses are input to the primary winding of a transformer and the secondary pulses, which may be at a higher or lower voltage than the input voltage, are rectified and regulated, the regulation often involving variation of the input pulse width. One very efficient DC-to-DC converter design is a full-bridge, resonant power converter. Such a converter is described in Steigerwald U.S. Pat. No. 4,864,479 issued Jul. 4, 1989. The converter of the Steigerwald patent is capable of operating at high frequencies, e.g. 1 MHz, and achieving high power densities. The Steigerwald converter employs a zero-voltage switching technique to produce the DC output voltage. Furthermore, zero voltage switching allows for a highly efficient conversion of power and reduced switching noise.

One way to obtain multiple output voltages from a DC-to-DC converter, such as the aforementioned full bridge resonant converter, is to provide additional windings on the output transformer. One main DC output voltage can be regulated according to the method taught in the '479 patent. In order to obtain auxiliary regulated output voltages, however, a high degree of coupling among all transformer windings is important. At high frequencies, tight coupling among all transformer windings can be difficult to achieve, resulting in output voltages that do not track closely enough. Thus it is desirable to have a regulation means for auxiliary outputs that is not closely coupled to the regulation means for the main output.

One means of regulating auxiliary DC output of a zero voltage switching power supply is described in Steigerwald, U.S. Pat. No. 5,038,264, issued Jun. 11, 1990. The '264 Steigerwald patent employs linear series-pass regulators to regulate the auxiliary voltages. A series-pass regulator, however, may be less efficient than desired, since power is wasted in the drop in voltage from the input to the output of the regulator. A more efficient scheme is desirable that would employ a switching regulator to regulate each auxiliary output and would advantageously synchronize the auxiliary regulation with the switching waveform of the secondary side of the main power transformer.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a DC-to-DC converter with a phase-shifted lossless resonant bridge conversion and regulation of a main DC output voltage and to provide regulation for at least one auxiliary secondary output voltage supplied by additional secondary windings of the transformer supplying the main output. According to another aspect, efficient lossless soft switching of the devices used to regulate the auxiliary output voltages is achieved. In one configuration, a high frequency, full bridge, resonant converter provides a main DC output voltage that is regulated by adjusting the phase shift of the input power to a main input transformer and at least one additional DC output voltage that is regulated on the secondary side of the power input transformer.

A full bridge resonant switching converter is operated substantially in the manner described in Steigerwald, '479 to efficiently produce a first DC output voltage. The magnetizing and leakage inductances of the high frequency transformer exchange energy between the transformer and the switching device output capacitances such that the energy stored in the device capacitances is returned to the DC source rather than dissipated in the device.

At least one additional secondary winding on the transformer produces a second set of pulsed DC voltages, the pulses being in phase with the pulses on the secondary winding supplying the main DC output voltage. Two switching devices are used for each auxiliary DC output to regulate each auxiliary output voltage by adjusting the length of the ON time of the pulses from the transformer's auxiliary secondary windings. Soft switching techniques are used to ensure that the switching devices turn on when the voltage across them is effectively zero.

An improved multiple output DC-to-DC converter includes a transformer with a primary and secondary windings wherein the transformer includes inherent inductances in series therewith. The converter comprises an inverter having a plurality of controllable switching devices Q1, Q2and Q3, Q4adapted to be connected in parallel across a DC supply. Each of the controllable switching devices includes an inherent diode portion D1-D4in inverse parallel therewith, and each of the controllable switching devices further includes an inherent capacitance C1-C4in parallel therewith. A first junction J1is between a first pair of the controllable switching devices Q1, Q2; and a second junction J2is between a second pair of the controllable switching devices Q3, Q4. The primary winding of the transformer is connected between the first and second junctions. The transformer includes a first secondary winding N2inductively coupled to the primary winding, the first secondary winding connected to a rectifying circuit supplying a first controllable DC voltage to a first load. A first control circuit is adapted to turn the first pair and the second pair of the controllable switching devices on and off at times which result in a substantially lossless transfer of energy between the inherent inductances and the inherent capacitances. The transformer includes a second secondary winding N3that is center-tapped and inductively coupled to the primary winding. The center tap of the second secondary winding forms a reference potential Vout2(-) for an auxiliary output voltage Vout2and a first end of the second secondary transformer winding N3(a) is connected to the anode side of a first diode D5and a second end of the second secondary transformer winding N3(b) is connected to the anode side of a second diode D6. The cathode end of the first diode connects to a first controllable switching device, and the cathode end of the second diode connects to a second controllable switching device; the other ends of the first and second switching devices forming a junction J3at which a second controllable DC output voltage Vout2is supplied to a second load. A second control circuit is adapted to turn on and off the first and second controllable switching devices at times that result in the regulation of the second controllable DC output voltage.

DETAILED DESCRIPTION

FIG. 1shows the schematic diagram of an exemplary embodiment of the invention.

An improved multiple output DC-to-DC converter is shown including a transformer T1with a primary N1and secondary N2, N3windings. Inductances relating to leakage effects and that are inherent in the transformer are depicted as L1-L4. The converter has an inverter for transforming the DC supply voltage30into a pulse-width modulated quasi-square wave. The inverter is comprised of controllable switching devices Q1, Q2and Q3, Q4, which in the embodiment shown are power Field Effect Transistors (FETs). Each of the field effect transistors has an inherent capacitance in parallel with its source and drain. The capacitances for each of the FETs are shown as C1-C4. Each FET also has an inherent diode, sometimes called a body diode in parallel with the source and drain. The diodes for each of the FETS are shown inFIG. 1as D1-D4.

The drain of FET Q1and the source of FET Q2are connected at a junction J1. The drain of FET Q3and the source of FET Q4are connected at a junction, J2. Junctions J1and J2are connected across the primary winding N1of transformer T1.

Controller U1is connected to Q1-Q4and has an input which is a feedback voltage80from the first regulated DC output, Vout1.

FETS Q1-Q4and controller U1switch the DC input voltage30to produce an alternating, quasi-square wave voltage Va across the primary windings of transformer N1. An oscilloscope trace of the primary winding voltage (140) is substantially shown inFIG. 2. The first DC output of the converter, Vout1is controlled in conventional fashion by varying the pulse width of the quasi-square wave drive voltage to N1. This operation is controlled by controller U1, which maintains the level of the signal Feedback1by controlling the on and off times of FETS Q1to Q4. Voltage Vout1is regulated by adjusting the width of the pulses supplying voltage to primary winding N1. This produces a quasi-square wave secondary voltage on windings N2and N3. This voltage is rectified by diodes D5-D8and filtered by inductor L5and capacitor C9to produce the first, or main, output voltage, Vout1. Vout1is fed back to U1via feedback connection80, which enables U1to adjust the on and off times of Q1-Q4to regulate Vout1to a predetermined desired level.

The switching of Q1-Q4is timed such that the inherent leakage inductance L1of the transformer winding N1and magnetizing inductance of winding N1(not shown) exchange energy between the transformer and the switching device output capacitances such that energy stored in the FET inherent capacitances C1-C4is returned to the DC source (30) rather than dissipated in the switching devices. This lossless resonant switching and the timing required to achieve this effect in a phase-shifted full bridge converter for the main output (Vout1) of the schematic shown inFIG. 1is known to those skilled in the art and can be accomplished as described in Steigerwald, U.S. Pat. No. 4,864,479.

Transformer T1also has a second secondary winding N3that is center-tapped and inductively coupled to the primary winding, N1. The second secondary winding also has associated inherent leakage inductances, shown inFIG. 1as L3and L4.

The center tap of the second secondary winding, N3c forms a reference potential Vout2(-) for an auxiliary output voltage Vout2. The first end N3aof secondary winding N3is connected to the anode side of diode D5and the second end N3bof secondary winding N3is connected to the anode side of a second diode D6;

The cathode of D5is connected to a controllable switching device Q5, which in the depicted embodiment is an FET and D5is connected to the source Q5. The cathode end of D6is connected to the source of FET Q6. The drains of FETs Q5and Q6form a junction J3, at which a second controllable DC output voltage Vout2is supplied to a second load through filter components L6, C11and C12.

Controller U2, which has as an input Feedback2(90) of Vout2, controls the switching of FETs Q5and Q6so as to regulate voltage Vout2.

In the exemplary embodiment depicted inFIG. 1, the regulation of Vout2is achieved as follows. A quasi square wave having the same duty cycle as that produced at primary windings N1(shown inFIG. 2at140) is also produced across windings N3aand N3b(shown inFIG. 2at120). This voltage is rectified by diodes D5and D6in conventional full-wave fashion, but only if gate-controlled switches (FETs) Q5and Q6are conducting. By delaying the turn on of Q5and Q6, a shorter portion of the quasi square wave is rectified and the output voltage is reduced. This delayed turn on is shown inFIG. 2at160, which is the time between the rising edge of the output of the voltage at N3a(trace120) and the output of Q5(trace130). Voltage regulation is achieved by feeding back the output voltage (Vout2) to a control circuit (U2) that drives Q5and Q6and varies the amount of shortening of the quasi square wave.

The switching of Q5and Q6is accomplished with at or near a zero voltage crossing across the devices, resulting in an efficient “soft switching” transition. This aspect is accomplished as follows. When the voltage across N3is such that the dotted ends of N3aand N3bjust become positive, the voltage will not be applied to the output of Q5until the device is turned on. After an appropriate leading-edge delay, U2will command Q5to turn on. Full current will not appear across Q5instantaneously, however, because the instant Q5is closed, no current will flow through the windings of N3due to the inherent inductance L3of the transformer. Thus the voltage across Q5immediately drops to zero before any current begins to build up, thus assuring that no power is lost in Q5during the switch transition. This is shown in the scope trace ofFIG. 3.

Q5will conduct until the voltage on N3aand N3breverses due to the operation of the primary side bridge, i.e. when voltage V(a) reverses (to voltage level125onFIG. 2). When this transition occurs, the un-dotted ends of N3aand N3bbecome positive. A negative voltage at N3awill cause current in Q5and D5to stop flowing (due to the blocking effect of D5) as shown in the scope trace ofFIG. 4. Thus, Q5can be turned off at any time thereafter without voltage across it, again a soft-switched transition. Meanwhile, Q6is turned on after an appropriate delay off the leading edge (165) to supply current to the load on this part of the quasi square wave cycle. As was the case for the turn on of Q5, this is also a lossless transition.

The output Vout2is regulated by adjusting the turn on delay for Q5and Q6. Because the switch transitions take place with effectively zero voltage across the switches, high efficiency operation at high switching frequencies is possible, thus maintaining the advantages of the Phase Shifted Resonant Bridge on the primary side of the transformer.

In yet another aspect, the interaction between the control loop for the main output voltage (effected through Feedback1and U1inFIG. 1) and the control loop for the auxiliary voltage (Feedback2and U2) can be minimized by timing the ON time for Q5and Q6off the trailing edge of the quasi-square wave produced at N1(shown inFIG. 2at170and180). In this manner, when the trailing edge the main voltage loop is modulated to regulate the main voltage (Vout1), the leading edge of the auxiliary output regulator will be delayed by an equivalent amount, which will leave a constant volt-second product at the output of the leading-edge modulator in the auxiliary voltage control loop.

Because the regulation of Vout2is achieved by shortening the quasi-square wave is produced at secondary winding N3, the winding ratios of the transformer must be such that there is sufficient voltage at N3to supply the required output Vout2, when the pulse width of the primary drive circuit is at its narrowest, accounting for the full range of pulse widths that the primary drive circuit will produce in order to regulate Vout1.