LMG3410 Datasheet, PDF (18/33 Pages) Texas Instruments – 600-V 12-A Single Channel GaN Power Stage

LMG3410
SNOSD10A – APRIL 2016 – REVISED JUNE 2016
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10 Power Supply Recommendations
The LMG3410 requires an unregulated 12-V supply to power its internal driver and fault protection circuitry. The
low-side supply can be supplied from the local controller supply. The high-side device's supply must come from
an isolated supply or bootstrap supply.
10.1 Using an Isolated Power Supply
Using an isolated power supply to power the high-side device has the advantage that it will work regardless of
continued power-stage switching or duty cycle. It can also power the high-side device before power-stage
switching begins, eliminating the power-loss concern of switching with an unpowered LMG3410 (see Startup and
Slew Rate with Bootstrap High-side Supply for details). Finally, a properly-selected isolated supply will contribute
fewer parasitics to the switching power stage, increasing power-stage efficiency. However, the isolated power
supply solution is larger and more expensive than the bootstrap solution.
The isolated supply can be constructed from an output of a flyback or FlyBuck™ converter, or using an isolated
power module. When using an unregulated supply, ensure that the input to the LMG3410 does not exceed the
maximum supply voltage. If necessary, a 18 V zener to clamp the VDD voltage supplied by the isolated power
converter. Minimizing the inter-winding capacitance of the isolated power supply or transformer is necessary to
reduce switching loss in hard-switched applications.
10.2 Using a Bootstrap Diode
When used in a half-bridge configuration, a floating supply is necessary for the top-side switch. Due to the
switching performance of LMG3410, a transformer-isolated power supply is recommended. With caution, a
bootstrap supply can be used with the recommendations in this section.
10.2.1 Diode Selection
LMG3410 has no reverse-recovery charge and little output charge. Hard-switched circuits using LMG3410 also
exhibit high voltage slew rates. A compatible bootstrap diode must exhibit low output charge and, if used in a
hard-switching circuit, very low reverse-recovery charge.
For soft-switching applications, the MCC UFM15PL ultra-fast silicon diode can be used. The output charge of 2.7
nC is small in comparison with the switching transistors, so it will have little influence on switching performance.
In a hard-switching application, the reverse recovery charge of the silicon diode may contribute an additional loss
to the circuit.
For hard-switched applications, a silicon carbide diode can be used to avoid reverse-recovery effects. The Cree
C3D1P7060Q SiC diode has an output charge of 4.5 nC and a reverse recovery charge of about 5 nC. There will
be some losses using this diode due to the output charge, but these will not dominate the switching stage’s
losses.
10.2.2 Managing the Bootstrap Voltage
In a synchronous buck, totem-pole PFC, or other converter where the low-side switch occasionally operates in
third-quadrant mode, it is important to consider the bootstrap supply. During the dead time, the bootstrap supply
charges through a path that includes the third-quadrant voltage drop of the low-side LMG3410. This third-
quadrant drop can be large, which may over-charge the bootstrap supply in certain conditions. The V_DD supply of
LMG3410 must not exceed 18 V in bootstrap operation.
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