Application Note

Application Note
Increasing Hold-Up Time in Switch Mode Power Supplies
Hold-up time
Hold-Up time is defined as the duration of time that a power supply's output will remain within regulated limits
following a loss of input power. For example, for a 5V output power supply with a ±10% (±0.5V) regulated
output, the hold-up time is measured from the time the input is turned “OFF” to the time when the output
voltage drops down to 4.5V. The hold-up time is measured at full load and under nominal line conditions.
In linear power supplies the time the output fails following the failure of the input is almost immediate. In
switching power supplies, energy is stored in the bulk (input electrolytic) capacitor providing a useable hold
up time to protect against transient power outages. Hold-up time is a function of the energy storage capability
of the power supply and the specific loading of the power supply. Hold-up time values of between 15 and 50
milliseconds are often required for today's power supply systems.
Figure 1 shows graphically the relationship between the shut off of the input voltage and the fall of the output
Input Voltage
Output Voltage
Turn On Time
Hold-Up Time
Figure 1: Turn on & Hold-Up
Time DC to DC Converter
Design considerations
When a power supply requires the capability of continued operation for a short period of time following a
momentary input power interruption, an external circuit providing additional capacitance can be easily
Both DC to DC converters and AC to DC converters can take advantage of external hold-up capacitors,
however, for AC-DC systems, the external hold-up circuitry interfaces with the internal DC bus of the power
supply. Access to the internal DC bus voltage must be available and details about the power supply
rectification and the DC bus is required to make these capacitance calculations
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Application Note
As the hold-up time of a power supply system is highly dependant on the end application and overall system
parameters, hold-up capacitance is normally not designed directly into the power supply module, but
designed as an external support circuit in the overall power supply system.
DC to DC converters
For DC to DC converters the calculation of hold up capacitor is straight forward.
Choldup ≥
2 x Pload x Tholdup
η x [(Vin_nom)2 - (Vin_dropout)2]
Equation 1
• Choldup is the minimum value of holdup capacitor
• Pload is the total power dissipated by the load in watts
• Tholdup is the desired hold-up time
• Vin_nom is the nominal input voltage at the time of shutdown
• Vin_dropout is the value of input voltage that defines the start of the hold-up time
• η is the efficiency of the power supply
Rearranging equation 1 to calculate the hold-up time for an existing hold-up capacitor:
η x [(Vin_nom)2 - (Vin_dropout)2] x Chold-up
Thold-up_max =
2 x Pload
Equation 2
External hold up time circuit connected to input DC to DC converter
When choosing a hold-up time capacitor, it is recommended to use capacitors with low Equivalent Series
Resistance (ESR) as well as high rated for high ripple current. When higher voltage is involved, it will be likely
necessary to use two or more capacitors in series and/or series-parallel configurations to achieve the
required total capacitance. The “cold start” temperature of the power supply must also be considered when
selecting capacitor types because many capacitors will lose a large amount of their capacitance at low
Voltage bleeding resistors and reverse polarity protection diodes should be included to provide a discharge
path for the capacitors, avoiding injury from stored charge, and protecting the capacitors from damage at
turn-on and turn-off.
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Application Note
Figure 2 illustrates a sample hold-up circuit with sets of series - parallel capacitors, using four capacitors of
the same value. The total holdup capacitance, C_Holdup is equal to the total value of Capacitance in the
circuit. Using four capacitors also provides margin for the capacitor breakdown voltage when used with high
value of Vin.
In addition, for this circuit a resistor (R_limit) is used to limit in-rush current during hold-up capacitor charging.
A typical value for R_limit is around 50 ohms.
Input Hold-Up Time Circuit
DC to DC
Figure 2: External Hold Up Time on Input
External hold up time circuit connected to output DC to DC converter
When the input of the converter is not accessible, the circuit in Figure 3 can be used to increase the hold-up
time. At turn-on, Q1 is off and C2 is not connected to the output. When the voltage at C1 reaches the
threshold of Q1, C2 starts to charge slowly towards VO. Q1 is a low RON N channel MOSFET and R1 D1 is
optional for discharging C1. If R1 D1 is used, the R1 C1 time constant must be much greater than R2 C1.
Also the discharge R1 C1 time constant must be much greater than the required hold-up time.
Figure 3: External Hold Up Time on Output
Notes: Information furnished is believed to be accurate and reliable. However, Aimtec Inc. assumes no responsibility for the consequences of use of
such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Aimtec Inc. Specifications mentioned in this publication are subject to change without notice. This
publication supersedes and replaces all information previously supplied. Aimtec Inc. Products are not authorized for use as critical components in life
support devices or systems without express written approval of Aimtec Inc.
© 2006 Aimtec Inc. – All Rights Reserved
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