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Posted on 29 June 2019

High Frequency Carrying Gate Drive

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A small simply circuitry makes it possible

For the “Synchronous bridge rectifier with microcontroller on the mains” was developed a special “high side carrying gate drive circuitry”. The developer and author decided to make it simple, fast and reliable at the same time.

By Milan Marjanovic, Texas Instruments

 

High side gate drive

High side gate drive is always needed if the driven switch is not referred to the ground. In this case the switching element must be connected to the bus rail, ‘on the high side’. This means, the reference point (normally emitter or source) is not any more on the quiet potential – gnd. It is now jumping, changing own potential from zero to the bus voltage. Because of these issues there are several possibilities to drive high side switch:

Gate driver overview

What is carrying gate drive?

Normally, if we generate or transform energy, we try to control this energy by power, voltage, current, etc. over some feedback loop. In some cases there is no need to control these values so precisely; it can be related to the load conditions and some fixed parameters which are defined in the system. This is the way every open loop system works. However, in our case, for the gate drive we need two stable conditions:

1. No excitation ⇒ zero gate voltage (or close to zero) without possibility for self excitation.
2. Excitation ⇒ Defined gate voltage, transients and voltage slope have to be done as fast as possible.

When we build a galvanic isolated gate drive, the simple way is to do this with a transformer. But if this has to be done for a low frequency (<1kHz), the transformer is going to be too bulky and too expensive. In our example we can modulate high frequency signal with a low frequency source - in this case as control signal. On this way the energy transfer can be realized down to 0Hz, using again small low cost high frequency transformer. This type of gate drive is called “high frequency carrying gate drive”, because this high frequency signal is carrying the energy for the gate excitation.

Main Requirements Table 2

Main requirements

This system will work as an open loop system, and as we mentioned, some parameters have to be defined by the system. To ensure that the gate voltage will stay in defined range, the supply voltage has to stay also in the same range.

Main Requirements

How is it working?

Figure 2 shows the built and tested schematic.

Built and tested schematic

INA_A & INB_A are push-pull control signals, coming from MSP430. The switching frequency is 200kHz.

UCC27324 (double gate driver) is well suited to make an additional current gain and voltage shifting.

C1 is preventing saturation of transformer T1 during eventually DC offset on driver’s output.

Transformer T1 provides the galvanic isolation and AC power flow. D2 & D5 is respectively performing the rectification and eventually voltage clamping.

Simulation result: rising edge drive_1 & drive_2: transformer primary side. Gate: Gate source voltage of the power MOSFET

Simulation result: falling edge drive_1 & drive_2: transformer primary side. Gate: Gate source voltage of the power MOSFET

Block circuitry D3, D4, R4, Q2, R2 are building a constant current drain of 10mA. This current will bias the base of Q1 if the voltage on gate is going to be higher then excitation voltage (in this case voltage across C4). This way the gate voltage is going to be discharged by a constant sink current of minimum 300mA. This current value is enough to discharge even high gate charge in very short time.

C3 is filtering the high frequency excitation voltage and protecting MOSFET re-igniting against high Miller currents. This can happen if the high dv/dt would be applied on drain. This high speed voltage change can produce very high capacitive current (Miller current) flowing into the gate, causing gate voltage increasing to threshold voltage. Due to this fact, the MOSFET can get uncontrolled conducting, which can cause an error.

Measurements result rising edge 10us/div.

R3 should discharge the rest of charge which is below voltage threshold of 1V (two times base-emitter voltage threshold, Q1&Q2). See the test results on Figures 3 and 4.

Measurements result falling edge 10us/div.

Conclusion

With a small simply circuitry is possible to realize fast isolated carrying gate drive. The circuitry is very efficient and reliable. It was possible to do very precise calculations, simulations and measurements on the real hardware. Simulation and real measurement matched very well, according to the test results.

 

 

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