DCP01B SERIES
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SBVS012C − DECEMBER 2000 − REVISED AUGUST 2005
Decoupling Ceramic Capacitors
All capacitors have losses due to their internal equivalent
series resistance (ESR), and to a lesser degree their
equivalent series inductance (ESL). Values for ESL are
not always easy to obtain. However, some manufacturers
provide graphs of Frequency versus Capacitor
Impedance. These will show the capacitors’ impedance
falling as frequency is increased (see Figure 4). As the
frequency is increased, the impedance will stop
decreasing and begin to rise. The point of minimum
impedance indicates the capacitors’ resonant frequency.
This frequency is where the components of capacitance
and inductance reactance are of equal magnitude. Beyond
this point, the capacitor is not effective as a capacitor.
Input Capacitor and the effects of ESR
If the input decoupling capacitor is not ceramic with
< 20mΩ ESR, then at the instant the power transistors
switch on, the voltage at the input pins will fall momentarily.
Should the voltage fall below approximately 4V, the DCP
will detect an under-voltage condition and switch the DCP
drive circuits to the off state. This is carried out as a
precaution against a genuine low input voltage condition
that could slow down or even stop the internal circuits from
operating correctly. This would result in the drive
transistors being turned on too long, causing saturation of
the transformer and destruction of the device.
Following detection of a low input voltage condition, the
device switches off the internal drive circuits until the input
voltage returns to a safe value. Then the device tries to
restart. If the input capacitor is still unable to maintain the
input voltage, shutdown recurs. This process is repeated
until the capacitor is charged sufficiently to start the device
correctly. Otherwise, the device will be caught up in a loop.
Normal startup should occur in approximately 1ms from
power being applied to the device. If a considerably longer
startup duration time is encountered, it is likely that either
(or both) the input supply or the capacitors are not
performing adequately.
For 5V to 15V input devices, a 2.2µF low-ESR ceramic
capacitor will ensure a good startup performance, and for
the remaining input voltage ranges, 0.47µF ceramic
capacitors are good. Tantalum capacitors are not
recommended, since most do not have low-ESR values
and will degrade performance. If tantalum capacitors must
be used, close attention must be paid to both the ESR and
voltage as derated by the vendor.
Z
X
L
0
f
O
Frequency
Where:
XC is the reactance due to the capacitance,
XL is the reactance due to the ESL
f
O
the resonant frequency
Z =
√
(XC
−
XL)
2
+ (ESR)
2
Figure 4. Capacitor Impedance vs Frequency
At f
O
, X
C
= X
L;
however, there is a 180° phase difference
resulting in cancellation of the imaginary component. The
resulting effect is that the impedance at the resonant point
is the real part of the complex impedance; namely, the
value of the ESR. The resonant frequency must be well
above the 800kHz switching frequency of the DCP and
DCVs.
The effect of the ESR is to cause a voltage drop within the
capacitor. The value of this voltage drop is simply the
product of the ESR and the transient load current, as
shown in Equation (1):
Output Ripple Calculation Example
DCP020505: Output voltage 5V, Output current 0.4A. At
full output power, the load resistor is 12.5Ω. Output
capacitor of 1µF, ESR of 0.1Ω. Capacitor discharge time
1% of 800kHz (ripple frequency):
t
DIS
= 0.0125µs
t = C
×
R
LOAD
t = 1
×
10
−6
×
12.5 = 12.5µs
V
DIS
= V
O
(1 − EXP(−t
DIS
/τ))
V
DIS
= 5mV
By contrast the voltage dropped due to the ESR:
V
ESR
= I
LOAD
×
ESR
V
ESR
= 40mV
Ripple voltage = 45mV
Clearly, increasing the capacitance will have a much
smaller effect on the output ripple voltage than reducing
the value of the ESR for the filter capacitor.
V
IN
+
V
PK
*
(ESR
Where:
I
TR
)
(1)
V
IN
is the voltage at the device input.
V
PK
is the maximum value of the voltage on the
capacitor during charge.
I
TR
is the transient load current.
The other factor that affects the performance is the value
of the capacitance. However, for the input and the full wave
outputs (single-output voltage devices), the ESR is the
dominant factor.
13
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