Finally, the magnetic core and isolated windings of the pulse transformer require a relatively large package. This is because transformers can deliver only ac signals since the core flux must be reset each half cycle to maintain a volt-second balance. Another limitation with pulse transformers is they may not work well in applications that require signals that have more than 50% duty cycle. This can potentially switch the gate on and off when not intended, leading to damage of the MOSFETs. A potential problem in this application can occur when large transient gate drive currents flow in the inductive coils, causing ringing.
An advantage of using a pulse transformer is that it does not require isolated power supplies to drive the secondary side MOSFETs. The gate driver in Figure 3 will differentially drive the primary of the pulse transformer, which has two windings on the secondary to drive each gate of a half bridge. A gate driver IC can be used to deliver the high currents needed for charging the capacitive MOSFET gates. A pulse transformer is an isolation transformer which can operate at speeds often needed for half-bridge gate driver applications (up to 1 MHz). Next, we will look at galvanic isolators which have a speed advantage over optocouplers due to lower propagation delays and more accurate timing. & amp amp amp lt img src=' ' alt='Figure 2'& amp amp amp gt įigure 2. To run an optocoupler to its maximum speed, the LED current needs to be increased to more than 10 mA, consuming more power, and reducing the lifetime and reliability of the optocoupler, especially in high temperature environments common in solar inverter and power supply applications. The response speed of an optocoupler is also limited due to the capacitance of the primary side light emitting diode (LED), and driving the output to speeds up to 1 MHz will be limited by its propagation delay (500 ns max) and slow rise and fall time (100 ns max).
This will increase the required deadtime between switching one channel off and turning the other channel on, reducing the efficiency. It should be noted that optocouplers are manufactured as a discrete device, even if two are packaged together, so they will have limitations in channel-to-channel matching. The gate driver circuit is often included in the same package as the optocoupler, and it is most common for there to be two separate optocoupler gate driver ICs to complete the isolated half bridge, which makes for a larger solution size. The next approach, shown in Figure 2, avoids problems with high-side to low-side interactions by using two optocouplers to establish galvanic isolation between the outputs. & amp amp amp lt img src=' ' alt='Figure 1'& amp amp amp gt įigure 1. When this happens, the high-side driver can latch up and become permanently damaged. Parasitic inductance in the circuit can cause the output voltage, V S, to go below ground during a low-side switching event.
Half bridge mosfet driver circuit drivers#
Another concern is that high voltage gate drivers do not have galvanic isolation and rely, instead, on junction isolation to separate the high-side drive voltage from the low-side drive voltage in the same IC.
One potential issue with this circuit is that there is only one isolated input channel, and it relies on the high voltage driver to have the needed matching in the timing between channels, and it also relies on the deadtime needed for the applications. The typical approach to implementing the isolated half-bridge gate drive function is to use an optocoupler for isolation, followed by a high voltage gate driver IC, as shown in Figure 1. This reduces the deadtime from one switch of the half bridge turning off before the second switch turns on. The high-and low-side drivers need very close matching of the timing characteristics to allow accurate and efficient switching. The isolated half-bridge driver’s function is to drive the gates of high- and low-side N-channel MOSFETs (or IGBTs) with a low output impedance to reduce the conduction losses, and a fast switching time to reduce the switching losses. These design concepts will be discussed in detail as this article explores the ability of isolated half-bridge gate driver solutions to provide high performance and a small solution size. Isolated half-bridge gate drivers are used in many applications that range from isolated dc-to-dc power supply modules where high power density and efficiency are required, to solar inverters where high isolation voltage and long-term reliability are critical. Design Fundamentals of Implementing an Isolated Half-Bridge Gate Driver