Designing a Gate Driver Circuit to Achieve Optimum Switching Performance in a Full-Bridge DC-DC Converter
By David Toro, Product Applications Engineer
A bridge converter is a type of DC-to-DC converter topology that employs four active switching components in a bridge configuration across a power transformer. A full-bridge converter is a commonly used configuration that has lower losses than a bridge rectifier and provides isolation while stepping up or down an input DC voltage. It also offers other functions, including reversing polarity and multiple simultaneous output voltages in applications such as server power supplies, telecom rectifiers, battery charging systems, and renewable energy systems. The essential operation involves switching on a pair of transistors for one-half cycle of a control waveform and a second pair during the other half-cycle. The converter is controlled using pulse-width modulation (PWM) signals to simultaneously switch on each leg as a pair or switch them independently in series. A gate driver IC is typically used to produce the high current required to drive the gate input of a high-power transistor like an IGBT or power MOSFET. This article discusses the critical design techniques and layout parameters when designing Diodes Incorporated’s (Diodes) DGD2190M gate driver ICs to drive each half of the primary side in a full-bridge switching converter. It also provides guidelines on selecting the discrete component values required for optimal circuit operation.
Bootstrap Component Selection
A bootstrap circuit is an essential part of a DC-DC converter. It provides a voltage above the main supply rail necessary to drive the gate of an N-channel MOSFET used as a high-side switch. A bootstrap is a charge pump circuit consisting of a bootstrap diode, a bootstrap capacitor, and a current-limiting resistor. This section explains how to select each of these.
Resistor Selection
In Figure 1, when a low-side MOSFET (Q2 or Q4) turns on, VS is connected to GND, and the bootstrap capacitors (CB1, CB2) begin to charge. When the high-side MOSFETs (Q1 or Q3) turn on, VS moves above VCC, and the charge on the bootstrap capacitor (CB) provides the current to drive the IC high-side gate driver. When the low-side switch turns on (when power is first applied), CB is charged for the first time through the bootstrap resistor (RB1 and RB2) and the bootstrap diode (DB1 and DB2). The highest charge current occurs at this time as typically CB is not fully discharged at each cycle during regular operation. Therefore, a bootstrap resistor is included in the circuit to limit the inrush current that charges CB when VS drops below VCC. This inrush current is largest during the first charge. Limiting inrush current is desirable to limit the size of noise spikes on VS and COM, which may cause shoot-through. The amplitude and duration of the inrush current are primarily determined by the component values of the bootstrap resistor and capacitor (as well as VCC). When selecting a value for a bootstrap resistor, the intention is to limit the inrush current while minimizing its effect on the RC charging time constant. Typically, values between 3Ω and 10Ω are sufficient to dampen the inrush current without affecting the bootstrap turn-on voltage.
Figure 1. The primary side of a full-bridge converter with switches driven by two DGD2190M gate driver ICs
Diode Selection
The selected bootstrap diode should be rated higher than the maximum rail voltage since the diode must block the full rail voltage and any spikes seen at the VS node. Its current rating is the product of the total charge required for the driver IC and the switching frequency. An ultrafast 1A recovery diode is recommended to minimize any delay in charging the bootstrap capacitor.
Capacitor Selection
The first step in choosing the bootstrap capacitor is determining the minimum guaranteed voltage drop (ΔVBS) when the high-side device is switched on. In other words, the minimum gate-source voltage (VGSmin) must be greater than the high-side circuit's undervoltage lockout (UVLO). The minimum bootstrap capacitor is calculated from:
CBmin ≥ QT /ΔVBS
Where QT is the total gate charge of the MOSFET, it is recommended that a minimum margin of 2-3 times the calculated value be used. Using values lower than this may result in overcharging the bootstrap capacitor, especially during negative supply voltage transients. Typically for power supply applications, values of CBS from 0.1µF to 2.2µF are used. Low equivalent series resistance (ESR) ceramic capacitors are also recommended, and these should be placed as close to the VB and VS pins as possible.
Gate-Switching Design Considerations
Selecting a Gate Resistor
The most crucial time in the gate drive is the turn-on and turn-off of a switch; performing this function quickly with minimal noise and ringing is critical. If this happens too quickly, a rise/fall time can cause unnecessary ringing and electromagnetic interference (EMI). However, too slow switching results in a rise/fall time that will increase switching losses. Considering the gate driver components for DGD2190M in Figure 2, with the careful selection of RG1 and RRG1, it is possible to selectively control the rise time and fall time of the gate drive to the switch. For equal switching times for the high side and low side, it is recommended that the gate driver components for the high side and low side are mirrored. For example, RRG1 = RRG2, DRG1 = DGR2, and RG1 = RG2.
Figure 2. Gate resistors for DGD2190M
Decoupling Capacitors
When considering the overall performance of the entire half-bridge, it is essential to use the appropriate high-voltage decoupling capacitors. For the best stability (best high-frequency performance), these should be small ceramic capacitors (for example, 1µF 450V) that are placed close to the drain of the switches at the half-bridge, and an electrolytic bulk capacitor should be included in the on-board power supply.
Matching a Gate Driver to a Switch
Drive Current and Turn-On Time
Driver current is a critical parameter for a gate driver IC, i.e., its ability to source current to the gate of a switch at turn-on, and to sink current from the gate of a switch at turn-off. For the DGD2190M, the drive currents are IO+ = 4.5A and IO- = 4.5A (typical). The time taken to turn on and off the switch using the DGD2190M can be calculated by:
t = Qg/ Io
For example, the datasheet for Diodes’ DGTD65T15H2TF, 650V IGBT specifies Qg = 61nC; so, for the DGD2190M, the calculated time is:
tr = tf = 14ns
Preventing Unexpected Shoot-Through
Unwanted MOSFET turn-on, caused by CGD x dVDS/dt, is often the cause of unexplained shoot-through in a half-bridge circuit (Figure 3). Depending on the ratio of the CGS to CGD, when the dVDS/dt across the low-side switch (Q2) occurs (i.e. when the high-side MOSFET turns on), a voltage can appear at the gate of the Q2, turning it on and causing shoot through. In effect, gate bouncing occurs, causing ringing on VS and the power ground. An external capacitor may be added to the gate source of the MOSFET (for example, 1nF), which increases the ratio of CGS to CGD to prevent this from occurring.
Figure 3. Unexpected shoot-through with dVDS/dt
Minimum Pulse Width
The DGD2190M has an RC filter on the input lines for added robustness in noisy environments. With a rising edge at the input to the gate driver; and then after the propagation delay of the IC, delay from the gate resistor, and rise time of the MOSFET; the half-bridge will turn on and produce bus voltage at the output. This switch turn-on produces significant system noise. Therefore, it is suggested to ensure a minimum pulse width at the input to the IC from the MCU so that turn-off occurs after this event. As a rule, this minimum pulse width should be twice the propagation delay of the driver. Hence for the DGD2190M, the minimum pulse recommended at the logic inputs is 280ns.
Current Booster Circuit
Figure 4 displays the DGD2190M in a standard gate driver configuration, while Figure 5 shows the typical response of source and sink current of 4.5A and 4.0A respectively when using a 47nF load capacitor. Gate resistors R1 and R2 are 1Ω.
Figure 4. DGD2190M with standard gate driver circuit
Figure 5. DGD2190M typical output current response with a 47nF load capacitor
It is possible to increase the output current capability to drive larger MOSFETs or IGBTs with the addition of a drive enhancement circuit. Below is an example of a current booster circuit for the low-side MOSFET using bipolar transistors ZXTP2012Z and ZXTN2010Z, providing a boost drive current controlled by the low-side output (LO) pin (Figure 6). Resistor R2 limits the base current of transistors Q1 and Q2 at turn on, and R5 at turn off. Gate resistors RGH and RGL are connected to the emitters of Q1 and Q2 and limit the gate current of M2. The diode D3 provides a fast turn-off path as long as R5 is less than R2.
Figure 6. DGD2190M gate driver circuit with current booster (low side)
The enhanced response of the gate driver circuit can be seen in Figure 7, with the peak source current increasing to 8.5A and peak sink current at 9.5A with RGH = RGL = 1Ω.
Figure 7. DGD2190M response with a current booster circuit
Layout Recommendations
The layout also plays a considerable role in circuit performance since unwanted noise coupling, unpredicted glitches, and abnormal operation may arise due to the poor layout of associated components. Figure 8 shows the parasitic inductances in the high-current path (LP1, LP2, LP3, LP4) caused by metal traces. Therefore, the length of the tracks in red should be minimized, the bootstrap capacitor and the decoupling capacitor (CD) should be placed as close to the IC as possible, and low ESR ceramic capacitors should be used. Finally, the gate resistors (RGH and RGL) and the sense resistor (RS) should be surface-mount devices. These recommendations can reduce parasitic effects due to PCB traces.
Figure 8. Layout suggestions for DGD2190M in a half-bridge; lines in red should be as short as possible
This article discussed critical design techniques and layout parameters when using the DGD2190M gate driver ICs by Diodes Incorporated, which drive each half of the primary side in a full-bridge switching converter. Guidelines were also provided for selecting discrete component values for optimal circuit operation. Diodes’ DGD21904M gate driver has the same functionality as the 8-pin DGD2190M but comes in an SO-14 package with a separate VSS pin. This can be used in applications requiring two separate grounds (power and logic).
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This article originally appeared in Electronic Product Design and Test (EPDT) on August, 2024.
