Electronic Design
A Few Brief Gate Drive Tricks Can Improve Your Design

A Few Brief Gate Drive Tricks Can Improve Your Design

We’ve all struggled with gate drive circuits. I’ve spent months in the lab trying to get textbook drive waveforms with the first Superjunction devices that were on the market. I couldn’t get enough current into them to turn them on fast enough, I couldn’t hold the gate in the off state with the monstrous charge coupling in from the miller capacitance, and I had untold oscillations.

At that point, I started questioning everything, because none of my assumptions were working. Finally I did come up with a good gate drive circuit, but it wasn’t what I expected at all. I started learning at that point, and I’ve been at it for quite a while. I’d like to share some of the tricks that I’ve stumbled across. This article will discuss these tricks in relation to the half-bridge circuit with the International Rectifier high-voltage IC (HVIC).

The Basics
HVIC is IR speak for the half-bridge and high/low-side power switch drivers with a high-side channel that can withstand high voltage. These parts came out in the mid-1990s as an alternative to gate drive transformers or optoisolators for driving high-side MOSFETs in offline converters. The IC features an input stage, which detects and processes the input signals, a low-side driver, and a high-side driver that is isolated from the rest of the IC in a Pwell that is processed into the silicon die that can withstand high voltage. For some drivers, this is 600 V. For others, it’s 1200 V.

Lastly, IR has a family targeted at automotive and telecom dc mains voltages that can withstand 200 V dc. IR also has drivers that explicitly do not allow the high-side and low-side switches to be on at the same time. We call these devices half-bridge drivers. The drivers that allow the aforementioned conditions are called high/low-side drivers.

I have seen 600-V drivers used on 24-V circuits and lots of other combinations. As long as the mains voltage is above the undervoltage lockout limits of the IC, this is perfectly acceptable. For the sake of economics, though, it is wise to use the most applicable voltage range.

When using the HVICs in a circuit, it is important to note that the low-side switch in the power converter must turn on to charge up the bootstrap capacitor and allow the high-side driver (Fig. 1) to turn on the high-side switch.

As a rule of thumb, IR’s drivers are specified in terms of maximum on-state and off-state current. In most cases, the off-state current is larger than the on-state current by a factor of two or so. This is simply due to the operation of most voltage-sourced inverter or forward converter power stages. The turn-off event needs to be as fast as possible to avoid switching loss.

In most half-bridge or full-bridge converters with inductive loads or substantial leakage inductances, the energy stored in this inductance will commutate the bridge so the turn-on event occurs with lower voltage across the switch. The turn-on event of a power switch can be a little slower.

With this sort of operation, the low-side switch in the totem pole output circuit of the HVIC needs to be physically larger than the high-side switch, thereby having lower on resistance for higher off-state current on the power switch.

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Choosing A Device
A smaller IC often will be used, with consequently smaller off-state and on-state currents to the power switch. I’ve seen this for many reasons, including familiarity, cost, and internal availability. It’s also possible to buffer the current of these smaller ICs to drive larger devices with the addition of a PNP and NPN emitter follower to buffer the outputs (Fig. 2a).

The low-VCE(SAT) (on state saturation voltage), high-gain bipolar junction transistors from Diodes Inc. are good choices for the emitter followers. I’ve used many of the ZXTC devices with great success, including the ZXTC2062 dual NPN/PNP.

The VCE of the devices needs to be higher than the 15-V power supply of the HVIC, and the max collector current should be greater than the max gate drive current required. This current buffer not only allows us to drive larger devices, it also can be used as a means to localize the drive currents when the HVIC is mounted more than an inch or two away from the power switches. The emitter follower needs to be as close to the power switch as possible, with local bypassing.

In some cases, the HVIC is far away from the devices that need to be driven, and perhaps the turn-on edge doesn’t need to be as fast. In these cases, an emitter follower is not needed. Rather, we can use a PNP device with some current steering to turn the low-side device off quickly. In this case, the PNP and related components need to be as close to the power switch as possible.

When the HVIC sinks current from the power switch, the BE junction of the PNP is forward biased, and most of the turn-off current flows through the PNP device to local return. In this case, the PNP and related components (Fig. 2b) need to be as close to the power switch as possible.

An alternate implementation of the fast turn-off trick uses a low-voltage N channel MOSFET instead of a PNP bipolar junction transistor (BJT). This circuit works best when the gate of the power switch is pulled negative when the device is turned off. International Rectifier app note AN978 discusses a couple of ideas for developing a negative bias on turn-off with HVICs (Fig. 2c).

In this circuit, the N channel MOSFET is connected across the GS or GE terminals of the power switch (MOSFET or IGBT, respectively). When the driver transitions to high, the N channel turn-off assist device is held off by the –1-VF voltage across its gate source terminals. When the driver transitions to low, the diode blocks and the gate is pulled positive with respect to the source through the output impedance of the HVIC.

Due to the common ground between the B+ and the IC return, this circuit can only be used in the high-side path when used with an HVIC. A circuit like this may be of use in an active clamp reset or two-transistor forward circuit where the high-side turn-off needs to be as fast as possible.

If you have negative bias available, both the PNP and the N channel MOSFET “turn-off assist” circuits not only can assist at turn-off, they also can sustain the off state on the power switch through any events that may occur immediately after turn-off, including false turn-on of the power switch through the Miller capacitance charging the gate capacitance up to a high enough voltage to cause switch through.

In general, the bootstrap diodes (D3, D6, and D9 in Figure 2) should have the same voltage rating as the power switch. A 1-A ultrafast diode will usually suffice. The steering diodes can be lower-voltage Schottkies.

Clearly, there are other ways to drive power switches in converters that have a high-side switch. Some methods use pulse transformers, optoisolators, and gate drive transformers. Due to the space restrictions on this article, I simply don’t have the room to discuss them in the detail they deserve.

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