3 Benefits of a Gate Drive Transformer

There are a few advantages to using a gate drive transformer. To summarize, gate drive transformers include the following:

• Provide galvanic isolation to the gate drive circuit
• Transfer signal and power, removing the need for a bias supply
• Enable easy negative gate bias
• Can enable a short propagation delay
• Scale voltage and current based on the turns ratio
• Allow more customization for extreme or unusual end applications

Of these benefits, one of the most notable is that the gate drive transformer eliminates the need for a bias supply. High-voltage half-bridge gate drive ICs require a floating supply to power the high-side driver. This is often done with a bootstrap circuit. In isolated gate drive ICs, a separate isolated bias supply is sometimes required to preserve the isolation barrier. The gate drive transformer can transfer the gate current from the primary side gate driver to the secondary side power switch. Because power is transferred through the gate drive transformer, there is no need for a bias supply on the secondary side. Isolated bias supplies add a significant cost to the system, so a gate drive transformer-based approach can reduce system cost.

Another benefit from gate drive transformers is the inherent negative bias provided by the push-pull topology. The PNP turn-off circuit in Figure 2-1 rectifies the bipolar gate drive signal from ±V­_DD to +V­_DD. However, omitting the PNP turn-off circuit allows for a -V­_DD bias during the off period. In some high-power systems, a negative bias is applied to the power switch to improve immunity to miller turn-on. When the switch node transitions between high and low states, a parasitic current is injected through the C_GD capacitance, also known as Miller capacitance. The injected current causes the gate voltage to increase, and can cause a false turn-on if the gate voltage exceeds the threshold voltage of the power switch. A negative bias increases the voltage difference between the gate voltage and threshold voltage, and results in higher immunity to the miller turn-on effect. Many SiC FET and IGBT data sheets recommend a negative bias to alleviate miller turn-on.

Gate drive transformers can also enable low propagation delay. The gate drive signal propagates through the transformer at effectively the speed of light. In terms of gate drive circuits, this delay can be treated as 0.

The total delay comes from the propagation delay of the low-side driver IC, and rise or fall time added by circuitry on the secondary side. In Figure 3-1, the driver propagation delay appears between input and the primary voltage. However, the secondary voltage starts to rise as soon as the primary voltage is applied. The low inherent propagation delay means that gate drive transformer circuits can achieve very low propagation delays compared with other options. Other isolation methods can often use modulation techniques such as on-off keying. Any modulation and demodulation circuits can add propagation delay to the system. There are digital isolator ICs available such as ISO6521 that have typical propagation delays as low as 11ns. However, a gate drive transformer-based circuit can still be used to achieve a very low propagation delay system in applications where a fast response is necessary.

The last major benefit offered by a gate drive transformer is easy voltage scaling. The turns ratio of the transformer results in a multiplier effect on the voltage. While many designers can opt for a 1:1 turns ratio transformer, different ratios can allow for flexibility in system design. For example, a 5V bus can be used on the primary with a 1:3 turns ratio to yield a 15V gate drive voltage on the secondary. However, when voltage is scaled up, current is scaled down proportionately. In this case, the primary current can be roughly three times larger than in a 1:1 transformer system. Overall, the ability to easily scale voltages can save DC/DC converters and improve overall system efficiency and flexibility.