Rainbow Electronics MAX5974D - Manual (Page 21)

MAX5974A/MAX5974B/MAX5974C/MAX5974D
Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
21
Active Clamp Circuit
Traditional clamp circuits prevent transformer saturation
by channeling the magnetizing current (I
M ) of the trans-
former onto a dissipative RC network. To improve effi-
ciency, the active clamp circuit recycles I
M between the
magnetizing inductance and clamp capacitor. V CLAMP
is given by:
S
CLAMP
V
V
1 D −
=
where V S is the voltage of the power source and D is
the duty cycle. To select n-channel and p-channel FETs
with adequate breakdown voltages, use the maximum
value of V
CLAMP . V CLAMP(MAX) occurs when the input
voltage is at its minimum and the duty cycle is at its
maximum. V
CLAMP(MAX-NORMAL) during normal opera-
tion is therefore:
S(MIN)
CLAMP(MAX-NORMAL)
P O
S S(MIN)
V
V
N V
1
N V
−
=
Ч
Ч
where V S(MIN) is the minimum voltage of the power
source, N P /N S is the primary to secondary turns ratio,
and V O is the output voltage. The clamp capacitor,
n-channel, and p-channel FETs must have breakdown
voltages exceeding this level.
If feed-forward maximum duty-cycle clamp is used then:
DCLMP MIN
MAX-FF
S DCLMP2
DCLMP1 DCLMP2
V V
D 1
2.43 2.43
V R
1
2.43 R R
 
= = −
 
 
 
= − ×  
+
 
Therefore, V CLAMP(MAX-FF) during feed-forward maxi-
mum duty clamp is:
( )
S
CLAMP(MAX-FF)
MAX FF
DCLMP1 DCLMP2
DCLMP2
V
V
1 D
2.43 R R
R
−
=
−
× +
=
The AUX driver controls the p-channel FET through a
level shifter. The level shifter consists of an RC network
(formed by C AUX and R AUX ) and diode D5, as shown
in the Typical Application Circuits. Choose R AUX and
C AUX so that the time constant exceeds 100/f SW . Diode
D5 is a small-signal diode with a voltage rating exceed-
ing 25V.
Additionally, C
CLAMP should be chosen such that the
complex poles formed with magnetizing inductance
(L
MAG ) and C CLAMP are 2x to 4x away from the loop
bandwidth:
BW
MAG CLAMP
1-D
3 f
2 L C
> ×
π ×
Bias Circuit
Optocoupler Feedback (MAX5974C/MAX5974D)
An in-phase tertiary winding is needed to power the bias
circuit when using optocoupler feedback. The voltage
across the tertiary V
T during the on-time is:
T
T OUT
S
N
V V
N
= ×
where V OUT is the output voltage and N T /N S is the turns
ratio from the tertiary to the secondary winding. Select the
turns ratio so that V
T is above the UVLO shutdown level
(7.5V max) by a margin determined by the holdup time
needed to “ride through” a brownout.
Coupled-Inductor Feedback (MAX5974A/MAX5974B)
When using coupled-inductor feedback, the power for
the devices can be taken from the coupled inductor dur-
ing the off-time. The voltage across the coupled induc-
tor, V
COUPLED , during the off-time is:
C
COUPLED OUT
O
N
V V
N
= ×
where V OUT is the output voltage and N C /N O is the
turns ratio from the coupled output to the main output
winding. Select the turns ratio so that V
COUPLED is
above the UVLO shutdown level (7.5V max) by a margin
determined by the holdup time needed to “ride through”
a brownout.
This voltage appears at the input of the devices, less
a diode drop. An RC network consisting of R
SNUB and
C
SNUB
is for damping the reverse recovery transients of
diode D6.