Patent Application
20140240007
This application claims the benefit and priority of Great Britain Patent Application No. 1303585.2 filed Feb. 28, 2013. The entire disclosure of the above application is incorporated herein by reference.
This application relates to improved drive circuits for power transistors and in particular to methods of limiting the dissipation of energy by a power transistor (for example an insulated gate bipolar transistor (IGBT)) during turn-on and turn-off, in particular by controlling transient voltages and currents when the power transistor is switched on or when the power transistor is switched off.
A power transistor, such as an IGBT or a power MOSFET, is a device primarily used as an electronic power switch. Power transistors such as IGBTs are highly efficient and fast switching. The transient behaviour of a power transistor when switched on or switched off is critical to its operating performance. During switch on and switch off of the power transistor, transient voltages contribute significantly to the electromagnetic interference (EMI) signature of the device and cause energy to be dissipated as heat, which negatively affects the efficiency of the power transistor.
The energy loss during switch on of a power transistor is referred to herein as Eon. The energy loss during switch off of a power transistor is referred to herein as Eoff.
Power circuits including power transistors require well designed drive circuits to minimise losses while efficiently driving a power transistor. For instance, European patent application No. EP 2306647 describes a drive circuit for a switching device in which a gate resistor is provided for adjusting speeds of turn-on and turn-off of a semiconductor switching device and a capacitor is connected in parallel with the resistor.
However the arrangement as shown in EP 2306647 has the disadvantage that the turn-on- and turn-off time may be too fast leading to uncontrolled transient performance.
The proposed circuit will now be described further, by way of example only, with reference to the accompanying drawings, in which:
a and 2b show an example of the switching characteristics of the drive circuit shown in
Turning to
In the example shown in
The turn-on drive circuit 2 comprises a first circuit 20 comprising a resistor Rcontrol in parallel with a capacitor Ccontrol and a second circuit 22 comprising a resistor RG. The first and second circuits 20, 22 are in series with each other in the drive path of the semiconductor switching device 4.
The input G of the IGBT can be represented as a variable capacitor whose value is dependent on the operating voltage and transient stage i.e. when current flows into the gate terminal, the gate emitter voltage (VGE) increases. When switching on the IGBT into an existing inductor current IL, the collector current Ic begins to rise when VGE increases beyond the threshold of the device. The rate at which the collector current Ic rises is related to the rate of change of voltage at the gate terminal by the device transconductance. When the collector current Ic reaches the inductor load current IL, the freewheel diode DF can begin to turn off and then block the voltage across the load Lload. The increase in blocking voltage is reflected to the IGBT as a reduction in the collector emitter voltage (VCE). The IGBT internal capacitance between gate and collector (CGC) must be supplied with gate current to allow the voltage to fall. Typically the gate current and hence switching speed may be controlled using a gate resistor (RG). The value of resistor RG is increased to find an acceptable EMI performance for the product by limiting the dVCE/dt. This results in an increase in the turn-on energy loss Eon.
The rapid change in VCE can have a significant impact on electromagnetic interference (EMI) such as conducted and radiated emissions. The described circuit presents a method which allows a small gate resistor RG to be used to minimise losses associated with the current rise, while offering control of dVce/dt over all or part of the voltage fall time by the Ccontrol/Rcontrol circuit.
As shown in
A resistor RControl inserted in parallel to the capacitor CControl is selected to restrict the flow of charge into the IGBT during the fall of the collector voltage VCE so reducing the fall rate. It is possible to reduce the dVCE/dt over the entire voltage range or a latter part of the fall time. Current flowing through RControl is required to completely turn on the IGBT ensuring low conduction losses and to also recharge CControl in preparation for the next switching cycle.
The value of RG controls the ramp-up rate of the collector current IC which significantly affects the switching losses Eon at turn-on. The steeper the rise in IC, the lower the switching losses. The values of Rcontrol and Ccontrol control the VCE drop after turn on.
In the case of the turn-on circuit 2, the turn-on circuit 2 comprises a resistor RControl in parallel with the capacitor CControl and a resistor RG in series with the resistor RControl and the capacitor CControl. This means that initially current will flow through the uncharged capacitor CControl (which acts initially like a short circuit) and through the resistor RG. Subsequently, as the capacitor CControl becomes fully charged, the current flows through both the resistor RControl and the resistor RG. The value of the resistor RG in series with the capacitor CControl is less than the value of the resistor RControl in parallel with the capacitor. For example typical values which may be used are 4.7 Ohms for RG and 10 Ohms for RControl although the actual values of RG and RControl will depend on the specific power transistor and other components used. The value of RG controls the ramp-up rate of the collector current Ic which significantly affects the switching losses Eon at turn-on. The value of RControl controls the flow of charge into the IGBT during the fall of the collector voltage VCE so reducing the fall rate.
The turn-on switching loss (Eon) can be determined by multiplying the instantaneous voltages and currents to find the instantaneous power then integrating over the switching time. In an effort to minimize the switching loss, it is desirable to reduce the gate resistance allowing charge to flow into the gate at an increased rate hence increasing the rate of current rise. This also has the effect of supplying more charge to discharge CGC rapidly hence increasing dVCE/dt.
As can be seen in
The turn-off process of an IGBT takes a finite duration of time during which energy is dissipated as heat loss. The proposed turn-off drive circuit should reduce the turn-off switching time and hence reduce the turn-off losses without increasing the radiated emissions.
This improved IGBT turn-off control can be achieved with the use of a single passive capacitor (C1 in
In
During turn off, the drive output of the optocoupler 8 is pulled negative. Current will flow from the IGBT gate terminal G through R2, D1, R1 and C1. Another current will also flow from C2, through D1, C1 and R1.
At the instance of switching, a large current will flow through R2, D1 and C1 rapidly discharging the gate capacitance to a voltage determined by the midpoint between C1 and C2, causing Vice to rise rapidly shortening the power loss time.
In the case of the turn-off circuit 3, the turn-off circuit 3 comprises a resistor R1 in parallel with the capacitor C1 and a resistor R2 in series with the resistor R1 and capacitor C1. This means that initially current will flow through the resistor R2 and the uncharged capacitor C1 (which acts initially like a short circuit). Subsequently, as the capacitor C1 becomes fully charged, the current flows through both the resistor R2 and the resistor R1. The value of the resistor R2 in series with the capacitor C1 is less than the value of the resistor R1 in parallel with the capacitor C1. For example typical values which may be used are 4.7 Ohms for R2 and 10 Ohms for R1 although the actual values of R2 and R1 will depend on the specific power transistor and other components used. The value of R2 controls the ramp-down rate of the emitter current IE which significantly affects the switching losses Eoff at turn-off. The value of R1 controls the flow of charge out of the IGBT during the rise of the collector voltage VCE so reducing the rise rate.
C1 is selected to ensure that the initial positive current flow through the capacitor will have reduced to zero or turned negative before the Vce voltage has risen to +DC (the positive supply rail). This forces the gate current IG to reduce by flowing through the series combination of R2 and R1 (a higher impedance). This reduction in gate current maintains a low value of dIc/dt minimizing the voltage overshoot due to parasitic inductance and will not increase the radiated emissions.
The peak source of the radiated emissions has been identified using a wavelet transform as shown in
The turn-off drive circuit 3 as shown in
The values of the capacitors and resistors discussed herein are in practice determined empirically for each circuit to be used.
Drive circuits as discussed herein allow radio frequency (RF) emissions to be better controlled and so assist in optimising RF noise versus switching time and losses and enable control of voltage overshoots and surges in the output voltage of a power transistor. The drive circuits are passive circuits with passive devices and do not require feedback of the output voltage or current of the power transistor for control.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1303585.2 | Feb 2013 | GB | national |
