Abstract:
A high-side driver circuit for a switching power supply, configured to translate a low-side switching control signal referenced to a first ground rail to a high-side switching control signal referenced to a second, high-side ground rail for driving a switching control connection of a power switching device, the high-side driver circuit including first and second inputs to receive first and second low-side switching control signals; a differential amplifier having a differential pair of inputs coupled to said first and second inputs and having an output, the differential amplifier having a ground connection for connection to said high-side ground rail and a power connection to receive a power supply from a second voltage supply; and an output coupled to said differential amplifier output to provide the high-side switching control signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. § 119 to United Kingdom Patent Application No. 0525619.3, filed Dec. 16, 2005, which is incorporated in its entirety by reference herein. This application also claims the benefit of U.S. Provisional Application No. 60/756,537, filed Jan. 5, 2006, which is incorporated in its entirety by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention is generally concerned with what are known in the art as high-side driver circuits, in particular for switching power supplies.  
         [0004]     2. Description of the Related Art  
         [0005]     Broadly speaking, a high-side driver is a circuit which transforms a low-side (low voltage) gate drive signal, generally referenced to a system and/or chip ground, to a high-side (high voltage) gate drive signal which is typically referenced to the mid-point of a half bridge. This mid-point has a potential which can vary between, for example, 0 and 500 volts above the system or chip/PCB (Printed Circuit Board) ground. Often the mid-point potential is rapidly varying over a range of several hundred volts. The high-side driver circuit provides a means of translating or communicating between the low (voltage)-side and the high (voltage)-side circuitry.  
         [0006]      FIG. 1  shows an example fluorescent lamp driver circuit  100  incorporating a high-side driver  110 .  
         [0007]     The lamp driver has a mains power supply input  102  to a rectifier and filter stage  104 , for example comprising a bridge rectifier and smoothing capacitors, which provides a low voltage supply to a control circuit  106 . The rectifier and filter stage  104  also provides a high voltage supply  109 , optionally via a PFC (Power Factor Control) stage  108 .  
         [0008]     The control circuit provides set and reset outputs to a pair of level shift transistors which provide input signals to high-side driver  110 . The control circuit also provides a low-side drive signal to a low-side gate drive circuit  114 . The high-side drive  110  drives a high-side switching transistor  116   a  and the low-side gate drive drives a low-side switching transistor  116   b . Transistors  116   a  and  116   b  form a half-bridge coupled between the high voltage supply and a system ground rail  118 . The high-side driver  110  is connected to a mid-point  120  of the half-bridge. This mid-point also provides a high (alternating current) voltage output to lamp and resonant capacitor  122  via a de blocking capacitor and resonant inductor  124 .  
         [0009]     In operation the low-side drive signal typically comprises a waveform with a duty cycle of around 40 percent. Similarly the set and reset signals are controlled so that the drive to the high-side transistor also has a duty cycle of around 40 percent. The low-side and high-side transistor drive waveforms are aligned so that the low-side transistor and high-side transistor are not on simultaneously, the less than 50 percent duty cycle waveforms providing a degree of tolerance in achieving this.  
         [0010]     A difficulty with design of the high-side driver is that the signal (usually in the form of a current) which is used to communicate between the low-side and high-side circuitry is easily confused by system noise, especially when the half-bridge is switching.  
         [0011]     Conventional solutions to this problem rely on either making the communicating signal very large relative to the interfering signals, with a consequent waste of power, or on using blanking signals which blank out the (regular) interfering signal. As the interfering signals happen at known times, in particular when the mid-point of the bridge is flying (transitioning) high or low, blanking pulses can be used to blank out these signals, but this approach also wastes power, is tricky to implement, and is prone to error. Background prior art can be found in: U.S. Pat. No. 4,994,955; U.S. Pat. No. 6,897,492; U.S. Pat. No. 6,781,422; U.S. Pat. No. 6,781,423; U.S. Pat. No. 5,274,274; U.S. Pat. No. 5,068,571; and WO2003/055072.  
       SUMMARY OF THE INVENTION  
       [0012]     According to a first aspect of the invention, there is therefore provided a high-side driver circuit for a switching power supply, the power supply having a switching control system operating from a first voltage supply and a power switching device configured to switch a second voltage supply higher than said first voltage supply, said first voltage supply having a first ground rail, said power switching device having a switching control connection for controlling switching of said second voltage supply, said high-side driver circuit being configured to translate a low-side switching control signal from said switching control system and referenced to said first ground rail to a high-side switching control signal referenced to a second, high-side ground rail for driving said switching control connection of said power switching device, said low-side switching control signal comprising a first signal for switching said power switching device on and a second signal for switching said power switching device off, said high-side driver circuit comprising: first and second inputs to receive said first and second low-side switching control signals; a differential amplifier having a differential pair of inputs coupled to said first and second inputs and having an output, said differential amplifier having a ground connection for connection to said high-side ground rail and a power connection to receive a power supply from said second voltage supply; and an output coupled to said differential amplifier output to provide said high-side switching control signal.  
         [0013]     As mentioned above, typically the second, high-side ground rail oscillates (because of the action of the power switching device) between around 0V and a positive voltage of 100V or more. By contrast the first (low) voltage supply typically operates at less than 20V, for example around 5V. The power switching device should therefore be suitable for switching a voltage of greater than 100V, and generally at a power of greater than 1 watt.  
         [0014]     In preferred embodiments a level shift circuit is coupled between the first and second inputs of the high-side driver and the pair of inputs of the differential amplifier. Such a level shift circuit may comprise a pair of level shift transistors, in particular connected to the first ground rail.  
         [0015]     The differential amplifier may have a single-ended output for driving the high-side transistor, but in preferred embodiments, the differential amplifier output comprises first and second differential outputs. These may then be used to drive a set-reset latch with inputs coupled to these differential amplifier outputs, an output of the latch being used to drive the high-side transistor.  
         [0016]     In some particularly preferred embodiments the latch has set and reset inputs which have a threshold voltage within 20 percent of a supply voltage Vdd of the high-side driver circuit, more particularly of the differential amplifier. That is the threshold voltages of the latch are preferably within 20 percent of a difference between the second voltage supply and the high-side ground rail.  
         [0017]     In preferred embodiments the differential amplifier has a tail connected to the power connection for the differential amplifier, more particularly to the second (high-side) voltage supply. The differential amplifier preferably comprises a differential pair of transistors both coupled to the tail of the differential amplifier, providing a pair of output arms. Preferably each output arm comprises a transistor connected as a resistive load (rather than a conventional current mirror-type load). Preferably each of these load transistors has a control (for example, gate) connection coupled to a fixed voltage supply, for example the high-side voltage supply rail. This helps to keep the differential mode gain of the differential amplifier low—which is advantageous because, although the differential mode gain would be larger with a current mirror-type load, the common mode gain would also be increased. Preferably the differential amplifier has a differential mode gain of less than 10 or even less than 5.  
         [0018]     In embodiments of the circuit, broadly speaking the use of a differential amplifier helps to reduce the vulnerability of the circuit to common mode signals which arise when the mid-point of the bridge of the circuit of  FIG. 1  is flying (transitioning) high or low.  
         [0019]     In some preferred embodiments the tail current of the differential amplifier is set by a current mirror output, an input current to the current mirror being set by a further transistor connected as a resistor. Preferably a control (for example gate) connection is connected to a fixed voltage, more preferably the same fixed voltage as that to which the resistive load transistors of the differential amplifier are connected, for example the high-side supply rail.  
         [0020]     In some preferred embodiments of the high-side driver circuit the inputs to the differential amplifier, more particularly to the differential pair of the differential amplifier, are clamped by respective voltage clamps. Each voltage clamp is preferably connected between a differential input and the high-side supply rail. The voltage clamp may comprise a zener diode or, in a process without zener diodes, by one or more forward-biased diodes in series in anti-parallel with a single reverse-biased diode.  
         [0021]     Preferably a resistor is connected across each voltage clamp or zener, to discharge its capacitance. For example, when one of the level-shift transistors turns on, providing an input to the differential amplifier, a voltage is developed across the voltage clamp (zener) diode and its capacitance is charged. When the input, more particularly the level-shift transistor, turns off the resistor discharges this capacitance, thus attenuating an erroneous differential mode input signal to the differential amplifier which would otherwise be present.  
         [0022]     Preferably the high-side driver circuit also includes a filter on the differential amplifier output to reduce or substantially remove noise which may arise from spurious differential mode signals.  
         [0023]     In certain embodiments, the invention also provides a switching power supply including a high-side driver circuit as described above. Where this switching power supply includes a half-bridge, the high-side ground rail may be coupled to a mid-point of this half-bridge. The switching power supply may be configured as a (dc) switch mode power supply, or a power supply for driving a (fluorescent) lamp load or some other type of switching power supply.  
         [0024]     In certain embodiments, the invention further provides a method of translating a low voltage control signal for a power switching device of a switching power supply into a control signal referenced to a ground which varies over a range of at least 100 volts with respect to voltage control signal as a consequence of said power device switching, the method comprising: inputting a differential control signal at a first voltage for controlling said power device switching; amplifying said differential control signal using a differential amplifier to provide a differential control signal output referenced to said varying ground.  
         [0025]     Preferably the differential control signal amplifying has a gain of less than 10, thus reducing the sensitivity of the method to common mode signals. The method may further comprise driving the power switching device using the differential control signal output, in particular converting the differential control signal output to a single-ended control signal, for example using a set-reset latch.  
         [0026]     In certain embodiments, the invention further provides a circuit for translating a low voltage control signal for a power switching device of a switching power supply into a control signal referenced to a ground which varies over a range of at least 100 volts with respect to said low voltage control signal as a consequence of said power device switching, the circuit comprising: means for inputting a differential control signal at a first voltage for controlling said power device switching; and means for amplifying said differential control signal using a differential amplifier to provide a differential control signal output referenced to said varying ground. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:  
         [0028]      FIG. 1  shows an example of a switching power supply driving a lamp load;  
         [0029]      FIG. 2  shows an embodiment of a high-side driver circuit according to an aspect of the invention;  
         [0030]      FIG. 3  shows and example set-reset latch circuit for use with the high-side driver circuit of  FIG. 2 ; and  
         [0031]      FIG. 4  shows an example of a switch mode power supply (SMPS) including the high-side driver circuit of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]     Referring to  FIG. 2 , this shows an example of a high-side driver circuit  200  which may be employed, for example, in the switching power supply of  FIG. 1 , as well as in other types of switching power supply.  
         [0033]     Referring to  FIG. 2 , two signals are used to communicate between the low voltage side and the high voltage side of the circuit. In this example these are “set”  202   a  and “reset”  202   b  signals which, in the example circuit of  FIG. 1 , turn the gate of the high-side device  116   a  in the half-bridge  116   a, b  on an off respectively.  
         [0034]     Two transistors  204   a, b  provide a level-shift function, having their source connections connected to a system ground rail  206  and their drain connections coupled (indirectly) to the high-side Vdd rail  208 . These transistors transmit the set and reset signals to the high-side circuit by turning on for brief periods of time.  
         [0035]     In  FIG. 2 , broadly speaking, the high-side part of the circuit is that drawn above high-side ground rail  210  (which in the example of  FIG. 1  is connected to the mid-point  120  of the half bridge  116   a, b ). In operation the high-side ground rail  210  moves up and down, sometimes by 300-400V or more (above system ground rail  206 ) but the high-side Vdd rail  208  always remains at Vdd above the ground rail  210 . Typically Vdd is less than 10V, for example around 5V.  
         [0036]     In operation, when the mid-point  120  of the half-bridge is flying high or low, common mode currents (that is, the same current in the “set” and “reset” nets) flow through the wires connecting the drains of the level shift devices to the high-side circuitry. These common-mode currents can be confused with the intended set and reset signals. Therefore, a differential amplifier, indicated by dashed box  212 , is employed in the high-side circuit to differentiate between differential mode signals, such as single pulses on the set or reset transistors, and common-mode signals, which occur as described above when the mid-point bridge is flying high or low. The differential amplifier has a differential (balanced) output  216   a, b  which is used to set or reset an S-R latch  216  which provides an output  218  to drive the gate of a high-side switching device such as high-side transistor  116   a  in the circuit of  FIG. 1 .  
         [0037]     Referring in more detail to the circuit of  FIG. 2 , the differential amplifier  212  comprises a differential pair of (in this example PMOS) transistors  220   a, b  with their source connections connected together to a current bias transistor M 4 . Transistor M 4  is the output of a current mirror, the input to which is formed by transistor M 0 . The current input to the current mirror, more particularly to transistor M 0  in line  222  is set by the resistance of transistor M 1 , which is connected as a resistor with its gate connected to the high-side Vdd rail  208 .  
         [0038]     Each of the transistors  220   a, b  of the differential pair has a respective load resistor  224   a, b , each formed by a transistor M 2 , M 3  with a gate connection connected to Vdd rail  208 . As mentioned above, by using a resistor load for each of the differential pair of transistors, rather than a current mirror load, the common-mode gain is reduced (albeit with the differential mode gain). If transistors M 1 , M 2  and M 3  are all the same size (width) then the same current will flow in line  222  as in the output arms of the differential pair and transistors M 4  and MO should therefore have a size ratio of M 4 :MO =2:1. Alternatively MO and M 4  could be the same size and M 1  half the size of M 2  and M 3  (M 2  and M 3  having the same size).  
         [0039]     Thus, broadly speaking, the gain of the differential amplifier is kept low by the use of resistive loads because the differential mode signals are large. The output of the differential amplifier for a given differential mode signal is set by the bias circuit (M 0  and M 1 ), the ratio of M 1  to the transistors M 2  and M 3  used as resistive loads in the differential amplifier circuit, and the ratio of the bias transistors M 0  and M 4 . This. ensures that the output of the differential amplifier remains constant for a wide variety of process comers, supply voltages and temperatures.  
         [0040]     Preferably the differential mode gain is set at a level which is just sufficient to give a logic-level output to drive the S-R latch  216  for a typical differential input signal, so that the common-mode gain is kept extremely low. This reduces the risk of the S-R latch  216  being accidentally set or reset when the differential amplifier sees common mode signals. More particularly, the gain of the differential amplifier is set to give a threshold voltage which is just enough to switch latch  216  in a worst case design scenario. This worst case depends upon the value of Vdd, and also on the operation of the voltage clamps (described below). In outline, in a typical worst case scenario the input to the differential amplifier is set by a potential divider comprising one of the resistors  226  and the resistance of the level shift transistor  204  (when in sufficient current flows through a zener diode  228  for it to perform its clamping action). For an example Vdd of 5V a minimum differential mode worst case input voltage may be, for example, around 4V, and the differential mode gain is chosen accordingly (given the threshold switching voltage of latch  216 .  
         [0041]     Despite the use of a differential amplifier, mismatch in the level-shift devices can cause differential mode signals to occur for brief periods when the mid-point of the bridge (line  210 ) is flying high or low. Therefore, preferably the output of the differential amplifier is filtered by MOS (Metal Oxide Semiconductor) capacitors  230   a, b  to remove these pulses. As can be seen from  FIG. 2 , preferably the circuit is implemented in MOS technology and either PMOS capacitors to Vdd or NMOS capacitors to ground may be employed, although the latter is preferable due to the reduced chance of coupling supply noise to the input of the S-R latch  216 . The filter should have a time constant of less than 1 microsecond, for example around  200  ns and the value of the capacitor can be determined from this and the effective output resistance of transistors M 2  and M 3 . A typical capacitance value is of order 2pF. Broadly speaking larger capacitance values (longer time constants) are better for noise immunity but it is also desirable to be able to provide short pulses to the level shift transistors  204 . The filtering should not be too great otherwise the desired output pulses will be filtered and will thus fail to set and reset latch  216 .  
         [0042]     Any conventional set-reset latch design may be employed in the circuit of  FIG. 2 . However it is preferable that the threshold voltages on lines  214   a, b  for setting and resetting the latch can be close to Vdd, for example, within 20 percent or even 10 percent of Vdd.  FIG. 3  shows an example S-R latch design which allows the threshold voltage to be biased well away from ground, thus increasing the circuits immunity to noise on the output of the differential amplifier. Referring again to  FIG. 2 , a zener diode  228   a, b  connects the input of each of the transistors of the differential pair to the high-side Vdd rail  208 . These zener diodes, load the level-shift transistors  204  and ensure that the inputs of the differential amplifier see the same voltage (with respect to the Vdd line). When a set or reset pulse is present, irrespective of the potential between the high-side circuit (ground rail  210  or Vdd rail  208 ) and system ground rail  206 . In a process without zener diodes, the zener diodes in the schematic may be replaced with, for example, four forward-biased diodes in series in anti-parallel with a single reverse-biased diode.  
         [0043]     Resistors  226   a, b  are connected in parallel with zener diodes  228   a, b  to ensure that the inputs of the differential amplifier see a high enough input voltage (that is, sufficiently below high-side Vdd rail  208  remembering that transistors  220   a, b  are PMOS devices) when the potential of the high-side Vdd supply is very low. This can occur, for example, when the mid-point of the bridge (and hence line  210 ) is close to or even below system ground  206  (because of the resonance action of the load, the high-side ground rail  210  can go one diode drop, approximately 0.7 V below system ground rail  206 ). A typical zener voltage is of order 5V to 10V; the resistors may have a value of order 100kΩ. As mentioned above, the resistors  226   a, b  discharge the voltage developed across a zener diode and stored on its capacitance when the corresponding level-shift transistor turns off, thus damping a potential spurious differential mode signal.  
         [0044]     When the high-side ground rail  210  is flying (transitioning) high the zener diodes  228  conduct current in reverse-bias (that is against the normal diode direction) to charge the capacitance of the level-shift transistors  204 , thus generating the above-described common-mode signals. When the high-side ground rail  210  is flying (transitioning) low the zener diodes conduct current in forward-bias direction to discharge the capacitance of the level-shift transistors (because this capacitance is now charged to a higher voltage than the high-side Vdd rail). In this latter case the generated common mode signal is above the high-side Vdd supply rail and hence neither input transistor of the differential amplifier is turned on. Thus when the high-side ground rail  210  is flying (transitioning) low there is substantially no danger of spurious triggering of the S-R latch  216  from common-mode signals.  
         [0045]      FIG. 4  shows an example of a switch mode power supply incorporating a high-side driver circuit as shown in  FIG. 2 , for improved reliability of transforming a low-side gate drive signal to a high-side gate drive signal.  
         [0046]     Broadly speaking we have described the use of a differential amplifier with low gain the high voltage side of a high-side driver to differentiate between true set-reset signals and common-mode signals due to switching taking place. Embodiments of the circuit provide a reduced power consumption and do not need circuitry to generate blanking pulses. A switching power supply with a lamp load and a dc switch mode powder supply circuit have been given as examples of applications of the above described high-side driver circuit, but the skilled person will recognise that many other applications exist for this circuit.  
         [0047]     No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.