Datasheet

IR2161(S) & (PbF)
www.irf.com 13
Since the load is resistive the current waveform will have a
sinusoidal envelope and so the peak can be easily deter-
mined taking into account that the current has a high fre-
quency component with an approximate 50% duty cycle:
Therefore:
For correct operation at maximum load the peak voltage
should be 0.4V.
The calculation can be simplified by combining the formulae,
Which can be simplified to:
Example
For a 100W convertor working from a 230VAC supply the
current sense resistor would need to be :
The nearest preferred value to this would be 0.33 Ohms.
The power dissipation in Rcs should also be considered
and is given by :
In this case :
It is important to bear in mind that the resistor must be rated
to handle this current in a high ambient temperature.
IMPORTANT NOTE
The filter resistor RL should be 1K, which is needed to
protect the CS input from negative going transients. CCS
should be 1nF and is also necessary to filter out switching
transients that can impair the operation of the shutdown
circuit.
Adaptive Dead Time
Because of the fact that the DC bus voltage varies during
the mains half cycle, the dead time may need to vary in
order to achieve soft switching. The IR2161 has an adap-
tive dead time circuit (ADT) that detects the point at which
the voltage at the half bridge slews to 0V (COM) and sets
the LO output high at this point. There is an internal sample
and hold system that allows approximately the same delay
to be used to set HO high after LO has gone low. This
reacts on a cycle-by-cycle basis of the oscillator and there-
fore will adjust the dead time as necessary regardless of
external conditions.
Figure 12, ADT when VS slews from VBUS to COM
AC
LOAD
RMSCS
V
P
I =
)(
)()(
22
RMSCSPKCS
II ×=
CSPKCSPKCS
RIV ×=
)()(
W062.033.0
230
100
2
=×
⎟
⎠
⎞
⎜
⎝
⎛
LOAD
CS
CS
P
V
R
⋅⋅
⋅
=
22
4.0
LOAD
AC
CS
P
V
R ⋅= 141.0
Ω=
×
324.0
100
230141.0
CS
AC
LOAD
CS
R
V
P
P ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
2