Unfortunately, using a transient voltage suppressor with
a higher voltage rating would not provide a sufficiently low
clamping voltage.
The waveforms R3 and R4 are with a 22 mF, 35V AVX
TPS type tantalum capacitor and a 22 mF, 30V Sanyo OS-
CON capacitor, respectively. With these two capacitors,
the transients have been brought to manageable levels.
However, these capacitors are bigger than the ceramic
capacitors and more than one capacitor is required in order
to meet the input ripple current requirements.
Table 1.2 Peak Voltages of Waveforms In Figure 1.3
TRACE C
IN
(mF) CAPACITOR TYPE V
IN
PEAK (V)
R1 22 Ceramic 40.8
R2 22 Ceramic with
30V TVS
32
R3 22 AVX, TPS Tantalum 33
R4 22 Sanyo OS-CON 35
Optimizing input capacitors
Waveforms in Figure 1.3 show how input transients vary
with the type of input capacitors used.
Optimizing the input capacitors requires clear under-
standing of what is happening during transients. Just as in
an ordinary resonant RLC circuit, the circuit in Figure 1.1
may have an underdamped, critically damped or over-
damped transient response.
Because of the objective to minimize the size of input
filter circuit, the resulting circuit is usually an underdamped
resonant tank. However, a critically damped circuit is actu-
ally required. A critically damped circuit will rise nicely to
the input voltage without voltage overshoots or ringing.
To keep the input filter design small, it is desirable to use
ceramic capacitors because of their high ripple current
ratings and low ESR. To start the design, the minimum
value of the input capacitor must first be determined. In
the example, it has been determined that a 22 m F, 35V
ceramic capacitor should be sufficient. The input transi-
ents generated with this capacitor are shown in the top
trace of Figure 1.4. Clearly, there will be a problem if
components that are rated for 30V are used.
To obtain optimum transient characteristic, the input
circuit has to be damped. The waveform R2 shows what
happens when another 22 mF ceramic capacitor with a
0.5 W resistor in series is added. The input voltage transient
is now nicely leveled off at 30V.
Critical damping can also be achieved by adding a capac-
itor of a type that already has high ESR (on the order of
0.5 W). The waveform R3 shows the transient response
when a 22 mF, 35V TPS type tantalum capacitor from
AVX is added across the input.
Table 1.3 Peak Voltages of Waveforms In Figure 1.4 with 22 mF
Input Ceramic Capacitor and Added Snubber
TRACE SNUBBER TYPE Vin PEAK (V)
R1 None 40.8
R2 22 mF Ceramic + 0.5 W In Series 30
R3 22 mF Tantalum AVX, TPS Series 33
R4 30V TVS, P6KE30A 35
Ch1 47 mF, 35V Aluminum
Electrolytic Capacitor
25
The waveform R4 shows the input voltage transient with
a 30V transient voltage suppressor for comparison.
Finally, an ideal waveform shown in Figure 1.4, bottom
trace (Ch1) is achieved. It also turns out that this is the
least expensive solution. The circuit uses a 47 mF, 35V alu-
minum electrolytic capacitor from Sanyo (35CV47AXA).
This capacitor has just the right value of capacitance and
ESR to provide critical damping of the 22 mF ceramic capac-
itor in conjunction with the 1 mH of input inductance. The
35CV47AXA has an ESR value of 0.44 W and an RMS
current rating of 230mA. Clearly, this capacitor could not
be used alone in an application with 1A to 2A of RMS ripple
current without the 22 mF ceramic capacitor. An additional
benefit is that this capacitor is very small, measuring just
6.3mm by 6mm.
Conclusion
Input voltage transients are a design issue that should not
be ignored. Design solutions for preventing input voltage
transients can be very simple and effective. If the solution
is properly applied, input capacitors can be minimized
and both cost and size minimized without sacrificing
performance.
[(Figure_4)TD$FIG]
Figure 1.4
*
Optimizing Input Circuit Waveforms for
Reduced Peak Voltage
SECTION ONE Power Management Tutorials
6