Emitter Drive Pin; Base Drive Pin - Power integrations LinkSwitch-4 LNK4*15D Series Application Note

Application Note
Constant Current Control by the CURRENT SENSE
Pin Average Voltage
The controller evaluates the average voltage across R
to control the maximum output current of the circuit. A current
control loop aims to limit the average voltage to V
limit, the voltage control loop is dominant, but at and beyond the
limit, the current control loop is dominant.
Average primary current
_
So by transformer primary to secondary turns ratio;
I =
OUT(CC)
However, the V supply and primary clamp take energy out of the
CC
transformer so an actual circuit will be a little lower than this
calculated value. It can be trimmed by R
this error remains fairly constant through production. So once
trimmed for a design, the maximum output current will remain
consistent across production.

EMITTER DRIVE Pin

Internally, the EMITTER DRIVE pin is connected to a pull down
MOSFET. This connects the switching BJT emitter to GND during the
'Charge' period of the switching cycle. This MOSFET must be capable
of conducting the peak primary current without incurring significant
power loss. There is also a pair of ESD protection diodes, one from
GROUND Pin to the EMITTER DRIVE pin and a second from the
EMITTER DRIVE pin to the VOLTAGE SUPPLY pin. This second diode
is used during the start-up cycle to charge the capacitor C
passing the BJT's emitter bootstrap current from the EMITTER DRIVE
to VOLTAGE SUPPLY pins.
The switching of this pin is synchronized with the BASE DRIVE pin to
enable rapid and safe, controlled switching of the BJT, see Figures 45
and Figure 46.

BASE DRIVE Pin

The BASE DRIVE pin supplies current through the switching BJT base
to turn it on quickly, but provide only just enough charge to keep the
BJT saturated whilst allowing it to be rapidly turned off. Like the
EMITTER DRIVE pin there is a pull down MOSFET , but this is used to
turn the BJT off. This too must be capable of conducting the peak
primary current as the primary current is passed through the BJT
base when the EMITTER DRIVE pin MOSFET is turned off. This
speeds up the turn off of the BJT.
There are four stages in the base drive cycle, see Figure 45 and
Figure 46;
1. A high initial level of base current for a short duration, the 'Force
On Pulse', amplitude I , duration t
F(ON)
2. A controlled level and duration of base drive based on the output
power demand. The level is proportional to the control voltage
and the duration is controlled by a feedback loop that aims to
keep the time between pulling the base low and the collector
rising by approximately 30 V to around 100 ns. Additional current
may be supplied by the SBD resistor via the SUPPLEMENTARY
BASE DRIVE pin, at this time for higher power designs using
SO-8 packaged devices.
3. A period where the BASE DRIVE pin is high impedance and the
BJT consumes the charge stored in the base region in order to
conduct the primary current.
4. BASE DRIVE pin is pulled low to turn the BJT off. The EMITTER
DRIVE pin simultaneously goes high impedance.
30
Rev. B 10/17
. This is used
CS
. Before this
CS(CC)
V
= CS(CC)
_ R
CS
N V
^ h
CS CC
P
N R
S CS
and once a design is fixed,
CS
by
VCC
.
F(ON)
V
CC
BD current
level control
Logic
BD
I
1
I
2
Logic
Logic
ED GND
BD GND
Figure 45. Simplified Base Drive Circuit.
I
BD
V
CC
I
ED
V
CE
V
ED
t t
1 2
Logic
BD
Logic
BD GND
Logic
ED GND
Figure 46. BJT Switching Cycle Waveforms.
AN-69
BD
ED
PI-8158-103116
b
a
t t t
3 4 5
ON
OFF
ON
OFF
ON
OFF
PI-8154-102816
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