MOSFETs and BJTs are important circuit elements in many systems requiring amplification, filtering, switching and continuous modulation of analog signals. Today’s power systems are using more advanced semiconductors in an effort to provide highly efficient power delivery, either with harmonic signals or high-bandwidth pulses. Materials in high electron mobility transistors (HEMTs), namely GaN and SiC, are responsible for enabling high efficiency in advanced power systems for applications like EVs and 5G.
The main circuits used in these advanced systems to provide highly stable power delivery and regulation are pulse driver circuits. Because these circuits will continue to play a greater role in power delivery, we’ll examine how these circuits are being implemented with more advanced HEMT components and some steps needed to ensure power stability.
HEMT pulse driver circuit designs
Pulse driver circuits in power systems, particularly those used as gate drives for power or motor control, are composed of a pulse width modulation (PWM) generator that modulates a switching element, which may need to operate at fast edge rates and high switching frequencies. Their role is to deliver pulses to a downstream load element with minimal power loss. In the context of pulse modulators in RF systems or power regulators, pulse driver circuits come in a variety of topologies:
- Single-ended pulse driver
- N-channel load switch or drain pulsing circuit
- Push-pull gate driver
- Half-bridge or full-bridge gate driver
Any of these pulse driver circuits may be isolated by passing the base pulse stream through a pulse or gate drive transformer.
The simplest topology is the single-ended pulse driver, which uses a single switching element. More complex topologies involve multiple switching elements and a control block used to synchronize FETs. When driven by the base pulse generator circuit (usually a PWM generator), the FETs need to be fully modulated into the ON state at high speed to provide the required switching action needed in advanced applications. The FETs used in pulse driver circuits should also have some important characteristics to ensure efficient power conversion and delivery. This is where HEMTs like GaN FETs outperform traditional silicon power MOSFETs.
Why use HEMTs in pulse driver circuits?
Semiconductor devices with high electron mobility, particularly transistors, are excellent for power conversion applications thanks to their low R-ON during operation. Today’s advanced HEMTs are designed with GaN, GaN on SiC, pure SiC or GaAs. In addition to high electron mobility and low R-ON values, these materials have high bandgap and higher thermal conductivity than Si, so they have lower parasitic gate capacitance values and can run at higher temperatures.
For power applications, these characteristics provide two important benefits: higher power handling limits and faster switching. While some pulse drive circuits and HEMT stages for lower-power applications are incorporated into integrated circuits with standard packaging, higher power systems must be built from discrete components. PWM drivers, HEMTs, passives and possibly transformers are placed in a larger circuit on the same board. This creates component selection and layout challenges that are often found in power supplies.
Challenges in pulse driver circuit designs
Pulse driver circuits and their switching elements follow a simple concept: they are effectively the same switching stages used in switching DC-DC converters, but without any rectification on the output. Therefore, they will be subject to many of the same problems surrounding noise, pulse fidelity, parasitics, and EMC. Faster switching speeds available in HEMT pulse driver circuits enable deeper modulation into the ON state with lower losses, however this also creates the aforementioned problems
The example above shows a simplified view of a push-pull pulse driver circuit. To isolate the design, a transformer with a coupling capacitor in series with the primary winding may be used. In the case of an isolated design, there may be parasitic underdamped oscillations superimposed on the output pulse if the LC resonance of these components is excited. If the load is inductive, there may also be some underdamped oscillation or resonance if there is any parasitic capacitance.
The latter case would be less common than the former case when traditional Si PWM drivers are used simply because the pulse bandwidths could be shorter. When HEMTs driven at fast edge rates (~10 ns or faster edges) are used, the resulting pulses will also be faster and will have larger bandwidth, making it easier to strongly excite underdamped oscillations in the design. These are normally suppressed by adding some damping, where a small amount of resistance (a few Ohms) is placed in series with the high-side leads of the HEMTs (Q1 and Q2 above), in series with any capacitors in the design, or all of the above.
Isolated pulse drivers
In isolated pulse drivers, where a transformer is used to provide galvanic isolation between the driver circuit and the load, there are two additional challenges to consider in the design:
- Preventing a DC voltage from forming across all sides of the transformer to prevent saturation
- Preventing the magnetizing inductance and coupling capacitance from resonating during switching or if the duty cycle changes quickly
In some cases, a tertiary winding on the primary side, or an optically coupled feedback loop from the secondary side, will be used with a measurement circuit to track pulse timing, level and edge rate. The PWM driver on the primary side can then track and adjust to changes or instabilities in these parameters.
If we wanted to use the above pulse driver for an application like battery charging, we would simply add DC rectification and a bank of capacitors on the output. These points should illustrate many of the challenges in pulse driver circuit design, particularly their correspondence to the similar challenges in high-voltage/high-current power regulators.