OPERATING
CONSIDERATONS
BC20 • BC20A
RIN is the value of the 2 external resistors used to reduce
gain.
The control circuit power supply, Vcc is internally bypassed
with a 0.1 µf ceramic. There is no additional bypassing inApex
test set ups, although it certainly wouldn’t be harmful.
Solving this equation for RFBI
FBI = 125000(GM)(V)( RS)/ (2.5(A)(V) - GM(RL)) - 10K
:
APPLICATION REFERENCES
R
For additional technical information please refer to the
following Application Notes:
AN 1: General Operating Considerations
AN 30: PWM Basics
If 125000(V)(RS) is large compared to RL(RFBI + 10K), not
always the case, then the equations simplify to:
Gm = A(RFBI + 10K)/(50000RS)
RFBI = 50000(RS)(GM)/A - 10K
25 kHz PWM, High Inductance Load
However the voltage gain of the PWM amplifier is twice as
high when 4 quadrant PWM is used. In this case:
25°C
500
Gm = 5(A)(V)(RFBI +10K)/(RL(RFBI +10K) + 250000(V)(RS))
400
300
200
100
0
Solving this equation for RFBI
:
RFBI = 250000(GM)(V)( RS)/(5(A)(V) - GM(RL)) - 10K
If 250000(V)(RS) is large compared to RL(RFBI +10K), not
always the case, then the equations simplify to the same
as in the 2 quadrant case:
GM = A(RFBI +10K)/(50000RS)
0
2
4
6
8
10 12 14 16 18 20 22
AMPERES
RFBI = (50000(RS)(GM)/A) - 10K
Figure 2: Continuous Current Safe Operating Area
100
GROUNDING AND BYPASSING
The BC20 output switches hundreds of volts and tens
of amperes with nano-second rise and fall times. Thus
care in bypassing and grounding is required to eliminate
noise in the system..
80
60
40
High voltage return and signal ground are electrically
isolated in the BC20. This allows connections which avoid
ground loops. However in order to avoid damaging offsets
between grounds on internal components internal back to
back schottky diodes are installed between signal ground
and high voltage return. So, at a minimum, signal ground
and high voltage must be tied together at one point in the
system. Clean operation has been obtained with single point
grounding, grounds tied together at the BC20, and with the
combination of single point grounding for dc with grounds
ac connected at the BC20 with a 1 µf capacitor. The system
designer should follow best practice for his system.
On the high voltage supply a switching regulator grade
electrolytic capacitor should be installed between high voltage
and high voltage return. This capacitor should be installed at
the BC20, with leads as short as practical. Apex recommends
10 µf per ampere of output current for this capacitor. The
voltage rating should withstand the highest transient voltage
on the high voltage supply; transients should not be allowed
exceed 450V for safe operation of the BC20. This is
required even if large value filter capacitors are in the
high voltage supply.
20
0
25
35
45
55
65
75
85
95
105
115
125
CASE TEMPERATURE, °C
Figure 3: Power Derating, Each Active IGBT
A 1 µf minimum ceramic capacitor with the same voltage
rating as the electrolytic should also be installed across
the high voltage supply. This capacitor should be installed
directly from the high voltage pin to the high voltage return
pin. (In our test set ups, this capacitor has a total lead length
of less than 2 inches.)
This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.
6APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739
BC20U REV C APRIL 2003
© 2003 Apex Microtechnology Corp.
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