Application Note 88 - Linear Technology

Application Note 88

AN88-1

A recent trend in the design of portable devices has been

to use ceramic capacitors to filter DC/DC converter inputs.

Ceramic capacitors are often chosen because of their

small size, low equivalent series resistance (ESR) and high

RMS current capability. Also, recently, designers have

been looking to ceramic capacitors due to shortages of

tantalum capacitors.

Unfortunately, using ceramic capacitors for input filtering

can cause problems. Applying a voltage step to a ceramic

capacitor causes a large current surge that stores energy

in the inductances of the power leads. A large voltage spike

is created when the stored energy is transferred from

these inductances into the ceramic capacitor. These volt-

age spikes can easily be twice the amplitude of the input

voltage step.

Plug In the Wall Adapter at Your Own Risk

The input voltage transient problem is related to the

power-up sequence. If the wall adapter is plugged into an

AC outlet and powered up first, plugging the wall adapter

output into a portable device can cause input voltage

transients that could damage the DC/DC converters inside

the device.

March 2001

Ceramic Input Capacitors Can Cause Overvoltage Transients

Goran Perica

, LTC and LT are registered trademarks of Linear Technology Corporation.

Figure 1. Block Diagram of Wall Adapter and Portable Device Connection

Building the Test Circuit

To illustrate the problem, a typical 24V wall adapter used

in notebook computer applications was connected to the

input of a typical notebook computer DC/DC converter.

The DC/DC converter used was a synchronous buck

converter that generates 3.3V from a 24V input.

The block diagram of the test setup is shown in Figure 1.

The inductor L

OUT

represents the lumped equivalent

inductance of the lead inductance and the output EMI filter

inductor found in some wall adapters. The output capaci-

tor in the wall adapter is usually on the order of 1000µF; for

our purposes, we can assume that it has low ESR—in the

10mΩ to 30mΩ range. The equivalent circuit of the wall

adapter and DC/DC converter interface is actually a series

resonant tank, with the dominant components being L

OUT

and the lumped ESR (the lumped ESR must include the

ESR of C

, the lead resistance and the resistance of L

OUT

The input capacitor, C

, must be a low ESR device,

capable of carrying the input ripple current. In a typical

notebook computer application, this capacitor is in the

range of 10µF to 100µF. The exact capacitor value depends

OUT

1000µF

35V

22µF

CERAMIC

M2 LOAD

AC INPUT

OUT

1µH to 10µH

SW1 DC/DC CONVERTERWALL ADAPTER

AN88 F01

OUTPUT CABLE

3 FEET TO 10 FEET

Application Note 88

AN88-2

on a number of factors but the main requirement is that it

must handle the input ripple current produced by the DC/

DC converter. The input ripple current is usually in the

range of 1A to 2A. Therefore, the required capacitors

would be either one 10µF to 22µF ceramic capacitor, two

to three 22µF tantalum capacitors or one to two 22µF OS-

CON capacitors.

Turning On the Switch

When switch SW1 in Figure 1 is turned on, the mayhem

starts. Since the wall adapter is already plugged in, there

is 24V across its low impedance output capacitor. On the

other hand, the input capacitor C

is at 0V potential. What

happens from t = 0s is pretty basic. The applied input

voltage will cause current to flow through L

OUT

. C

will

begin charging and the voltage across C

will ramp up

toward the 24V input voltage. Once the voltage across C

has reached the output voltage of the wall adapter, the

energy stored in L

OUT

will raise the voltage across C

further above 24V. The voltage across C

will eventually

reach its peak and will then fall back to 24V. The voltage

across C

may ring for some time around the 24V value.

The actual waveform will depend on the circuit elements.

If you intend to run this circuit simulation, keep in mind

that the real-life circuit elements are very seldom linear

under transient conditions. For example, the capacitors

may undergo a change of capacitance (Y5V ceramic

capacitors will loose 80% of the initial capacitance under

rated input voltage). Also, the ESR of input capacitors will

depend on the rise time of the waveform. The inductance

of EMI-suppressing inductors may also drop during tran-

sients due to the saturation of the magnetic material.

Testing a Portable Application

Input voltage transients with typical values of C

and L

OUT

used in notebook computer applications are shown in Fig-

ure 2. Figure 2 shows input voltage transients for C

val-

ues of 10µF and 22µF with L

OUT

values of 1µH and 10µH.

The top waveform shows the worst-case transient, with a

10µF capacitor and 1µH inductor. The voltage across C

peaks at 57.2V with a 24V DC input. The DC/DC converter

may not survive repeated exposure to 57.2V.

The waveform with 10µF and 10µH (trace R2) looks a bit

better. The peak is still around 50V. The flat part of the

waveform R2

following the peak indicates that the

synchronous MOSFET M1, inside of the DC/DC converter

in Figure 1, is avalanching and taking the energy hit. Traces

R3 and R4 peak at around 41V and are for a 22µF capacitor

with 1µH and 10µH inductors, respectively.

Figure 2. Input Voltage Transients Across Ceramic Capacitors

Table 1. Peak Voltages of Waveforms In Figure 2

TRACE L

(µH) C

(µF) V

PEAK (V)

CH1 1 10 57.2

R2 10 10 50

R3 1 22 41

R4 10 22 41

Input Voltage Transients with Different Input Elements

Different types of input capacitors will result in different

transient voltage waveforms, as shown in Figure 3. The

reference waveform for 22µF capacitor and 1µH inductor

is shown in the top trace (R1); it peaks at 40.8V.

The waveform R2 in Figure 3 shows what happens when

a transient voltage suppressor is added across the input.

The input voltage transient is clamped but not eliminated.

It is very hard to set the voltage transient’s breakdown

voltage low enough to protect the DC/DC converter and

far enough from the operating DC level of the input

source (24V). The transient voltage suppressor P6KE30A

that was used was too close to starting to conduct at 24V.

Application Note 88

AN88-3

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 22µF, 35V AVX TPS

type tantalum capacitor and a 22µF, 30V Sanyo OS-CON

capacitor, respectively. With these two capacitors, the

transients have been brought to manageable levels. How-

ever, these capacitors are bigger than the ceramic capaci-

tors and more than one capacitor is required in order to

meet the input ripple current requirements.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

actually 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 22µF, 35V

ceramic capacitor should be sufficient. The input tran-

sients generated with this capacitor are shown in the top

trace of Figure 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 22µF ceramic capacitor with a

0.5Ω resistor in series is added. The input voltage tran-

sient is now nicely leveled off at 30V.

Critical damping can also be achieved by adding a capaci-

tor of a type that already has high ESR (on the order of

0.5Ω). The waveform R3 shows the transient response

when a 22µF, 35V TPS type tantalum capacitor from AVX

is added across the input.

Figure 3. Input Transients with Different Input Components

Table 2. Peak Voltages of Waveforms In Figure 3

TRACE C

(µF) CAPACITOR TYPE V

PEAK (V)

R1 22 Ceramic 40.8

R2 22 Ceramic 32

with 30V TVS

R3 22 AVX, TPS 33

Tantalum

R4 22 Sanyo OS-CON 35

Optimizing Input Capacitors

Waveforms in Figure 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

may have an underdamped, critically damped or

overdamped 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

Figure 4. Optimizing Input Circuit Waveforms

for Reduced Peak Voltage

Table 3. Peak Voltages of Waveforms In Figure 4 with 22µF Input

Ceramic Capacitor and Added Snubber

TRACE SNUBBER TYPE V

PEAK (V)

R1 None 40.8

R2 22µF Ceramic + 0.5Ω In Series 30

R3 22µF Tantalum AVX, TPS Series 33

R4 30V TVS, P6KE30A 35

Ch1 47µF, 35V Aluminum 25

Electrolytic Capacitor

Application Note 88

AN88-4

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The waveform R4 shows the input voltage transient with

a 30V transient voltage suppressor for comparison.

Finally, an ideal waveform shown in Figure 4, bottom trace

(Ch1) is achieved. It also turns out that this is the least

expensive solution. The circuit uses a 47µF, 35V alumi-

num electrolytic capacitor from Sanyo (35CV47AXA). This

capacitor has just the right value of capacitance and ESR

to provide critical damping of the 22µF ceramic capacitor

in conjunction with the 1µH of input inductance. The

35CV47AXA has an ESR value of 0.44Ω 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 22µF 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.