ISL6754_14 Datasheet, PDF (11/19 Pages) Intersil Corporation – ZVS Full-Bridge PWM Controller with Adjustable Synchronous Rectifier Control

ISL6754
The EA available on the ISL6754 may also be used as the
voltage EA for the voltage feedback control loop rather than
the current EA as described above. An external op-amp may
be used as either the current or voltage EA providing the
circuit is not allowed to source current into VERR. The
external EA must only sink current, which may be
accomplished by adding a diode in series with its output.
The 4x gain of the sample and hold buffer allows a range of
150 - 1000mV peak on the CS signal, depending on the
resistor divider placed on I_OUT. The overall bandwidth of the
average current loop is determined by the integrating current
EA compensation and the divider on I_OUT.
CHANNEL 1 (YELLOW): OUTLL
CHANNEL 3 (BLUE): CS
CHANNEL 2 (RED): OUTLR
CHANNEL 4 (GREEN): IOUT
FIGURE 6. DYNAMIC BEHAVIOR OF CS vs I_OUT
The average current signal on I_OUT remains accurate
provided the output inductor current remains continuous
(CCM operation). Once the inductor current becomes
discontinuous (DCM operation), I_OUT represents 1/2 the
peak inductor current rather than the average current. This
occurs because the sample and hold circuitry is active only
during the on time of the switching cycle. It is unable to
detect when the inductor current reaches zero during the off
time.
If average overcurrent limit is desired, I_OUT may be used
with the error amplifier of the ISL6754. Typically I_OUT is
divided down and filtered as required to achieve the desired
amplitude. The resulting signal is input to the current error
amplifier (IEA). The IEA is similar to the voltage EA found in
most PWM controllers, except it cannot source current.
Instead, VERR has a separate internal 1mA pull-up current
source.
Configure the IEA as an integrating (Type I) amplifier using
the internal 0.6V reference. The voltage applied at FB is
integrated against the 0.6V reference. The resulting signal,
VERR, is applied to the PWM comparator where it is
compared to the sawtooth voltage on RAMP. If FB is less
than 0.6V, the IEA will be open loop (can’t source current),
VERR will be at a level determined by the voltage loop, and
the duty cycle is unaffected. As the output load increases,
I_OUT will increase, and the voltage applied to FB will
increase until it reaches 0.6V. At this point the IEA will
reduce VERR as required to maintain the output current at
the level that corresponds to the 0.6V reference. When the
output current again drops below the average current limit
threshold, the IEA returns to an open loop condition, and the
duty cycle is again controlled by the voltage loop.
The average current control loop behaves much the same
as the voltage control loop found in typical power supplies
except it regulates current rather than voltage.
C10
150 - 1000 mV
R6
1
2 VERR
3
ISL6754
4
5
6
7 FB
-
0.6V +
8
9 CS
S&H
4x
10 IOUT
R5
20 VREF
19 SS
18 VDD
17 OUTLL
16 OUTLR
15 OUTUL
14 OUTUR
13 N/C
12 GND
11 GND
R4
FIGURE 7. AVERAGE OVERCURRENT IMPLEMENTATION
The current EA cross-over frequency, assuming R6 >>
(R4||R5), is:
f_CO
=
-----------------¹------------------
2π ⋅ R6 ⋅ C10
Hz
(EQ. 6)
where f_CO is the cross-over frequency. A capacitor in parallel
with R4 may be used to provide a double-pole roll-off.
The average current loop bandwidth is normally set to be
much less than the switching frequency, typically less than
5kHz and often as slow as a few hundred hertz or less. This
is especially useful if the application experiences large
surges. The average current loop can be set to the steady
state overcurrent threshold and have a time response that is
longer than the required transient. The peak current limit can
be set higher than the expected transient so that it does not
interfere with the transient, but still protects for short-term
larger faults. In essence a 2-stage overcurrent response is
possible.
The peak overcurrent behavior is similar to most other PWM
controllers. If the peak current exceeds 1.0V, the active
output pulse is terminated immediately.
11
FN6754.1
September 29, 2008