Gate drive transformer specifications and applications

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A gate drive transformer is a transformer optimized for delivering rectangular electrical pulses with fast rise and fall times to activate or deactivate switching devices. It handles low power but high peak current to drive the gate of the power switch. Power ratings range from µW to several KW.

Gate drive transformers provide floating power for power semiconductors as well as level shifting of switching signals, eliminating a separate floating power supply. It can be used to directly drive the gate of a power switch (MOSFET/IGBT) or it can be used to isolate only the control signal and then apply it to the gate driver IC. Additionally, it provides impedance matching.

When operating at high switching frequencies (>100 KHz), the design and construction of a high isolation gate drive transformer requires careful consideration to avoid the adverse effects of parasitic components (leakage inductance and distributed capacitance)

Gate drive transformers may also be called pulse transformers, trigger transformers, broadband transformers or signal transformers. The distinction is mainly based on the actual use of the transformer: where the transformer is used to directly drive the gate of a power device it is called a gate drive transformer; if it is used only as a means to transmit a rectangular voltage signal/pulse to the gate of a semiconductor it is called a pulse transformer. In general, however, a pulse transformer transfers current/voltage pulses from the primary/generating side of the circuit to the secondary/load side of the circuit, while maintaining its shape and other characteristics. If a transformer pulse initiates some action or event, it can be called triggering the transformer.

Transformers have at least two windings (primary and secondary) that help with isolation, an important property. The turns ratio between primary and secondary also allows for voltage scaling, but is usually not required.

Basic circuit
The basic circuit of a transformer-based isolated gate drive is relatively large. In addition to the transformer, it also includes related reset components, such as DC blocking capacitor C, primary resistor R, gate resistor Rg, back-to-back Zener diode, etc. It can be divided into the following circuits:

1. IC direct drive type

Direct driving of this power IC is the most common and simplest driving method.



Using this method, we should pay attention to several parameters and their effects.

Check the power IC manual first for the maximum peak drive current, as different IC chips have different drive capabilities.

Secondly, check the parasitic capacitance of the MOSFET, such as C1, C2 and C3 in the figure. If the capacitance is large, the energy required to turn on the MOS tube is also relatively large. If the power IC does not have enough peak drive current, the transistor will turn on at a slower rate.

If the drive capability is insufficient, high frequency oscillation may occur on the rising edge, and even reducing Rg in Figure 1 will not solve the problem! Factors such as IC drive capability, MOSFET parasitic capacitance, and MOSFET switching speed will also affect the selection of drive resistors, so Rg cannot be infinitely reduced.

2. The push-pull output circuit enhances the drive

The function of the driving circuit is to increase the current supply capability and quickly complete the charging process of the gate capacitor input. This topology increases the turn-on time, but reduces the turn-off time, and the switch can be turned on quickly to avoid high-frequency oscillation on the rising edge.



3. The drive circuit accelerates the turn-off of the MOS tube

At the moment of turning off, the drive circuit can provide a path with the lowest possible impedance, so that the capacitance between the gate and the source of the MOSFET can be quickly discharged to ensure that the switch tube can be turned off quickly.

In order to ensure the rapid discharge of the capacitance C2 between the gate and the source, a Rg2 and a diode D1 are connected in parallel with the Rg1. Among them, D1 usually adopts a fast recovery diode, which shortens the turn-off time and reduces the turn-off loss; the function of Rg2 is to prevent the power IC from being burned out due to excessive current when it is turned off.



The totem pole circuit can also speed up the shutdown. When the driving capability of the power IC is sufficient, the circuit in Figure 2 can be improved to the following form.
It is very common to use a triode to discharge the electricity of the GS capacitor. If the emitter of Q1 has no resistance, the capacitance between the gate and the source will be short-circuited when the PNP transistor is turned on. crossover loss at small turn-off.


As shown in Figure 4, because of the existence of the triode, the capacitive current between the gate and the source will not be discharged directly through the power supply IC, which improves the reliability of the circuit.

4. The transformer drive circuit accelerates the turn-off of the MOS tube

In order to meet the requirements of driving high-side MOS transistors, as shown in Figure 5, a transformer driver is usually used, and sometimes it is also used for safety isolation.
The purpose of using R1 is to suppress the parasitic inductance on the PCB board and C1 to form LC oscillation, and its design purpose is to isolate DC, pass AC, while preventing core saturation.


Our engineers are experts who are well equipped to help customers select the most appropriate transformer for their unique project needs.

Whether clients need a Gate Drive model or a different transformer, we partner with every customer to help them find the best unit for their specifications.

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To learn more about our Gate Drive Transformers or to request a custom quote, please contact us directly.