M51995AP Datasheet, PDF (23/27 Pages) Mitsubishi Electric Semiconductor – SWITCHING REGULATOR CONTROL

MITSUBISHI (Dig./Ana. INTERFACE)
M51995AP/FP
SWITCHING REGULATOR CONTROL
Under the condition of current limiting operation,the output
current I2 continues as shown in Fig.33.So the output voltage
depends on the product of the input primary voltage VIN and the
on-duty.
If the third winding polarity is positive ,the Vcc depends on
VIN,so it is concluded that the smoothed voltage of VOUT
terminal depends on the output DC voltage of the SMPS.
So the sharp current limiting characteristics will be got,if the
VOUT voltage if feed back to VF terminal through low pass filter
as shown in Fig.35.
M51995A
VOUT
RVFFB
VF
CVFFB
Fig.35 Feed back loop through low pass filter from
VOUT to VF terminal
It is recommended to use 15kΩ for RVFFB,and 10,000pF for
CVFFB in Fig.35.
Fig.36 shows how to control the knee point where the frequency
becomes decrease.
FROM
VOUT
TO VF
FROM
VOUT
TO VF
FROM
VOUT
TO VF
TO MAKE THE KNEE
POINT HIGH
TO MAKE THE KNEE
POINT LOW
Fig.36 How to control the knee point
(b)In case of fly back system
The DC output voltage of SMPS depends on the Vcc voltage of
type M51995A when the polarity of the third winding is negative
and the system is fly back.So the operation of type M51995A
will stop when the Vcc becomes lower than "Operation-stop
voltage" of M51995A when the DC output voltage of SMPS
decreases under specified value at over load condition.
POINT THAT Vcc VOLTAGE
OR THIRD WINDING
VOLTAGE DECREASES
UNDER "OPERATION-STOP
VOLTAGE"
DC OUTPUT CURRENT
Fig.37 Over current limiting curve on fly back system
However,the M51995A will non-operate and operate
intermittently,as the Vcc voltage rises in accordance with the
decrease of Icc current.
The fly back system has the constant output power
characteristics as shown in Fig.37 when the peak primary
current and the operating frequency are constant.
To control the increase of DC output current,the operating
frequency is decreased using the characteristics of VF terminal
when the over current limiting function begins to operate.
The voltage which mode by dividing the Vcc is applied to VF
terminal as shown in Fig.38,as the induced third winding voltage
depends on the DC output voltage of SMPS.
15kΩ or less is recommended for R2 in Fig.38,it is noticed that
the current flows through R1 and R2 will superpose on the
Icc(START) current.
If the R1 is connected to Cvcc2 in Fig.27,the current flows
through R1 and R2 is independent of the Icc(START).
Vcc
COLLECTOR
M51995A
VF
R1
CVcc
R2
Fig.38 Circuit diagram to make knee point low on
fly back system
(c)Application circuit to keep the non-operating condition
when over load current condition will continue for
specified duration
The CT terminal voltage will begin to rise and the capacitor
connected to CT terminal will be charged-up,if the current
limiting function starts,and VF terminal voltage decreases below
VTHTIME(~3V).
If the charged-up CT terminal voltage is applied to OVP terminal
through the level-shifter consisted of buffer transistor and
resistor,it makes type M51995A keep non-operating condition.
( 23 / 27 )