ISL78223 Datasheet, PDF (13/20 Pages) Intersil Corporation – ZVS Full-Bridge PWM Controller with Adjustable Synchronous Rectifier Control

ISL78223
Voltage Feed Forward Operation
Voltage feed forward is a technique used to regulate the output
voltage for changes in input voltage without the intervention of
the control loop. Voltage feed forward is implemented in voltage-
mode control loops, but is redundant and unnecessary in peak
current-mode control loops.
Voltage feed forward operates by modulating the sawtooth ramp
in direct proportion to the input voltage. Figure 10 demonstrates
the concept.
VIN
ERROR VOLTAGE
RAMP
The charging time of the ramp capacitor is:
t
=
–R3
⋅
C7
⋅
⎛
ln ⎜ 1
–
V_{-----R----A----M-----P----(--P----E---A----K----)⎟⎞}
S
⎝
V_{IN(MIN) âŽ} 
(EQ. 7)
For optimum performance, the maximum value of the capacitor
should be limited to 10nF. The maximum DC current through the
resistor should be limited to 2mA maximum. For example, if the
oscillator frequency is 400kHz, the minimum input voltage is
300V, and a 4.7nF ramp capacitor is selected, the value of the
resistor can be determined by rearranging Equation 7.
R3 = -----------------------------------–---^t----------------------------------- = -----------------–---²---^.--⁵-----^â‹---¹---⁰----^–--⁶------------------
C7
⋅
⎛
ln ⎜1
–
-^V----^R----^A----^M-----^P----⁽--^P----^E---^A----^K----⁾⎟⎞
⎝
V_{IN(MIN )) âŽ} 
4.7 ⋅ 10^–9 ⋅ ln _⎝⎛1 – ₃----₀₁---₀--_⎠⎞
= 159 kΩ
(EQ. 8)
CT
OUTLL, LR
FIGURE 10. VOLTAGE FEED FORWARD BEHAVIOR
Input voltage feed forward may be implemented using the RAMP
input. An RC network connected between the input voltage and
ground, as shown in Figure 11, generates a voltage ramp whose
charging rate varies with the amplitude of the source voltage. At
the termination of the active output pulse, RAMP is discharged to
ground so that a repetitive sawtooth waveform is created. The
RAMP waveform is compared to the VERR voltage to determine
duty cycle. The selection of the RC components depends upon
the desired input voltage operating range and the frequency of
the oscillator. In typical applications, the RC components are
selected so that the ramp amplitude reaches 1.0V at minimum
input voltage within the duration of one half-cycle.
VIN
R3
C7
1
20
2
19
3
18
4
17
ISL78223
5
16
6
15
7
14
8 RAMP
13
9
12
10
GND 11
FIGURE 11. VOLTAGE FEED FORWARD CONTROL
where t is equal to the oscillator period minus the deadtime. If
the deadtime is short relative to the oscillator period, it can be
ignored for this calculation.
If feed forward operation is not desired, the RC network may be
connected to V_REF rather than the input voltage. Alternatively, a
resistor divider from CTBUF may be used as the sawtooth signal.
Regardless, a sawtooth waveform must be generated on RAMP
as it is required for proper PWM operation.
Gate Drive
The ISL78223 outputs are capable of sourcing and sinking 10mA
(at rated VOH, VOL) and are intended to be used in conjunction
with integrated FET drivers or discrete bipolar totem pole drivers.
The typical on resistance of the outputs is 50Ω.
Slope Compensation
Peak current-mode control requires slope compensation to
improve noise immunity, particularly at lighter loads, and to
prevent current loop instability, particularly for duty cycles
greater than 50%. Slope compensation may be accomplished by
summing an external ramp with the current feedback signal or by
subtracting the external ramp from the voltage feedback error
signal. Adding the external ramp to the current feedback signal is
the more popular method.
From the small signal current-mode model [1] it can be shown
that the naturally-sampled modulator gain, Fm, without slope
compensation, is:
Fm = ---------¹----------
SnTsw
(EQ. 9)
where Sn is the slope of the sawtooth signal and Tsw is the
duration of the half-cycle. When an external ramp is added, the
modulator gain becomes:
Fm = ------------------¹-------------------- = -------------¹--------------
(Sn + Se)Tsw m_cSnTsw
(EQ. 10)
where Se is slope of the external ramp and
m_c
=
1 + ^S-----^e--
Sn
(EQ. 11)
13
FN7936.1
January 2, 2013