P
Paul E. Schoen
- Jan 1, 1970
- 0
I just recently received eight SMT PC boards for a boost converter for 12
VDC nominal to 750 mA into a series high power LED string that may drop 24
to 40 VDC. It uses a Microchip 16F684 with 100 kHz PWM output, a TC1410
driver, and a Fairchild HUF75645 MOSFET, with a 10 uH inductor and a 100V
6A Schottky to drive the LEDs. In my prototype, the TC1410 shorted to Vdd
when I raised the input voltage to about 16 VDC with a poorly filtered
supply, and I figured it probably had voltage surges above its 20V maximum.
I replaced the IC with a 3 transistor driver, and it worked fine ever
since.
When testing the new board, the PWM seemed to work OK for an instant, and
then the TC1410 failed. Several others also failed, sometimes with output
shorted to ground, sometimes to Vdd. I added a 100 ohm resistor to the
output, and it failed again. I added anothe 100 ohm resistor to the Vdd
supply, and it survived, but did not properly drive the MOSFET.
The HUF75645 has a gate capacitance of 3800 pF, which is much higher than
the 500 pF for which the driver is characterized, but its internal
resistance of 16 ohms should have limited the output current to less than 1
ampere. It is rated at 500 mA, so I suspect this is the problem. However,
even a smaller MOSFET with 350-400 pF will also draw the same peak current,
although for a much shorter time. Simulating it in LTspice shows only about
100 mW total gate power for the 3800 pF gate. I have ordered some TC1413
drivers, rated at 3 amps, and characterized for 1500 pF, but they have 2.7
ohm resistance, so they would see pulse current of 6 amps at 16 VDC. The
app notes show direct connection to the MOSFET gate.
I can add 20-30 ohms to the gate and still get reasonable switching times,
while limiting the peak current to 500 mA for the existing device. However,
I'm fairly certain that the one driver failed with 100 ohms. The Vdd supply
is isolated from the main 10-16 VDC supply with a diode, and bypassed with
100 uF and 1 uF. The main supply is bypassed with 0.47 uF. I am very
concerned about reliability, as these boards will be potted in an assembly
that will not be repairable. It is a high power LED diving flashlight that
will have a parts cost of well over $100.
I'm waiting on a response from Microchip, but I am concerned that these
drivers may have reliability problems. I am considering redesign of the
boards to use the simple three-transistor driver that has been working for
a while in my rough prototype. Any suggestions?
Thanks,
Paul
VDC nominal to 750 mA into a series high power LED string that may drop 24
to 40 VDC. It uses a Microchip 16F684 with 100 kHz PWM output, a TC1410
driver, and a Fairchild HUF75645 MOSFET, with a 10 uH inductor and a 100V
6A Schottky to drive the LEDs. In my prototype, the TC1410 shorted to Vdd
when I raised the input voltage to about 16 VDC with a poorly filtered
supply, and I figured it probably had voltage surges above its 20V maximum.
I replaced the IC with a 3 transistor driver, and it worked fine ever
since.
When testing the new board, the PWM seemed to work OK for an instant, and
then the TC1410 failed. Several others also failed, sometimes with output
shorted to ground, sometimes to Vdd. I added a 100 ohm resistor to the
output, and it failed again. I added anothe 100 ohm resistor to the Vdd
supply, and it survived, but did not properly drive the MOSFET.
The HUF75645 has a gate capacitance of 3800 pF, which is much higher than
the 500 pF for which the driver is characterized, but its internal
resistance of 16 ohms should have limited the output current to less than 1
ampere. It is rated at 500 mA, so I suspect this is the problem. However,
even a smaller MOSFET with 350-400 pF will also draw the same peak current,
although for a much shorter time. Simulating it in LTspice shows only about
100 mW total gate power for the 3800 pF gate. I have ordered some TC1413
drivers, rated at 3 amps, and characterized for 1500 pF, but they have 2.7
ohm resistance, so they would see pulse current of 6 amps at 16 VDC. The
app notes show direct connection to the MOSFET gate.
I can add 20-30 ohms to the gate and still get reasonable switching times,
while limiting the peak current to 500 mA for the existing device. However,
I'm fairly certain that the one driver failed with 100 ohms. The Vdd supply
is isolated from the main 10-16 VDC supply with a diode, and bypassed with
100 uF and 1 uF. The main supply is bypassed with 0.47 uF. I am very
concerned about reliability, as these boards will be potted in an assembly
that will not be repairable. It is a high power LED diving flashlight that
will have a parts cost of well over $100.
I'm waiting on a response from Microchip, but I am concerned that these
drivers may have reliability problems. I am considering redesign of the
boards to use the simple three-transistor driver that has been working for
a while in my rough prototype. Any suggestions?
Thanks,
Paul