This article was posted on 08/01/2006.
Circuit protection for power supplies
How to select the right parts to keep your designs safe and reliable
BY JAMES COLBY
Des Plaines, IL
When looking at consumer and industrial applications today, it is easy to focus on the functionality of the device and take for granted the reliability of the power system. However, design engineers must consider that the features that help create their competitive advantage are at risk due to internal and external electrical threats to their systems.
Since power-supply circuits are susceptible to many electrical threats, the designer should take the time to choose the correct protection components.
A nearby thunderstorm, the switching of large inductive loads on the power grid, and surge transients all present threats that can disable the application. Coupled with internally created fault currents (due to components like capacitors or FETs reaching end-of-life), these factors can present safety hazards that behoove the designer to invest time into the selection of the best circuit-protection device for that application.
Electrical threats and solutions
Threats that originate in the power system include voltage transients such as lightning and load-switching events. Threats that originate within the power supply circuitry include short-circuit events and overload conditions (fault currents).
On the primary side of the circuit, a typical solution for surge transients is a metal oxide varistor (MOV), which clamps the incoming surge to a level that allows the power supply components to survive and continue functioning. Key MOV selection criteria include working voltage, energy-handling capability, and peak-pulse-current capability.
The MOV's working voltage is the voltage level at which the device stands off without breaking over (becoming conductive). This is typically the first parameter determined and is a straightforward process.
To specify the working voltage, the application's line voltage is increased by 20% to allow for voltage swells that can occur on the power system. This is done to ensure that the MOV responds to transient events and not to temporary conditions in the system.
As an example, assume that the line voltage is 120 Vac. Then, the minimum working voltage parameter that should be considered is 120 Vac x 1.20 = 144 Vac.
Since MOVs are not typically rated at 144 Vac, an MOV with a working voltage above this value should be selected. So for a 120-Vac line voltage, an MOV with a working voltage of 150 Vac should be selected.
The peak-pulse-current rating of an MOV is a measure of the maximum transient current it can handle without experiencing a shift in its electrical characteristics. MOV vendors typically provide multiple series of MOVs, and the peak-pulse-current characteristic is a key parameter, specified as a value for a given waveform (typically 8 x 20 �s) and in a chart form that allows an engineer to determine how many pulses a particular MOV is expected to survive based on peak pulse current, pulse width, and number of pulses.
With respect to safety on the primary circuit, a fuse is used to disrupt the current and power flow into the power supply in the event of a fault current condition. A fuse acts as an intentionally weak link in the circuit and interrupts the fault current before charring or smoking takes place, and before the circuit board is damaged.
The selected fuse should activate at a level low enough to protect the circuit, but high enough not to activate during normal startup of the application�otherwise known as nuisance tripping.
Of particular concern are in-rush currents. These currents are created when nonlinear components (capacitors and inductors) charge up as the application is turned on. These pulses can be many times the magnitude of normal operating currents, but are typically very short-lived (tens to hundreds of microseconds).
The first step in selecting the fuse is to determine the normal operating current of the circuit, as well as the ambient temperature in the area of the circuit where the fuse will be located. Lastly, fuses certified to UL standards must also be rerated down by 25% to properly apply them to the circuit. Here, the following equation can be used:
min fuse rating = max operating current/(UL fuse rerating x thermal derating factor)
For example, if a power supply draws 5 A of current, and the temperature is expected to be 80°C, then
Min fuse rating = 5.0/0.75 x 0.95 = 7.02 A
Since a fuse rated exactly at 7.02 A is not available, the fuse with the next closest rating greater than the calculated amperage rating is used. In this case, the selected fuse rating would be 8.00 A. Now that the amperage rating has been selected, the next step is to ensure that the fuse will not experience a nuisance trip during the power supply's expected lifetime.
To do this, it is necessary to capture a scope trace of the in-rush current generated during power supply power-up and calculate the melting energy (also known as the i2t value) of this in-rush current. This is typically a double-exponential curve of short duration.
The data of interest are the peak current and the duration of the in-rush current event. It is also necessary to estimate the number of power-up cycles the power supply is expected to experience over its working lifetime.
Armed with this data, the appropriate equation is selected by comparing the waveform of the in-rush current with datasheets or reference material provided by the fuse supplier. In this case, it is assumed that the in-rush current indeed demonstrates a double exponential waveform. Thus, the following equation is used:
Melting i2t = (1/2) x ipeak2 x time/pulse-cycling withstand ratio
For example, if the in-rush current pulse is measured to be 500 A, the duration of the pulse is 120 s, and the expected number of power-up cycles is 10,000, then the minimum i2t value of the fuse is
Melting i2t = (1/2) x (500)2 x 120 x 10-6/0.29 = 51.72 A2s
Now, all the information can be put together. It has been calculated that a fuse with an 8.0-A rating is needed, as well as a minimum requirement for melting i2t to be 51.72 A2s. Power supplies typically use glass cartridge fuses.
Two additional considerations may be appropriate for some designs. In the first case, there may be a regulatory requirement such as UL 1414 to ensure electrical isolation of the ac input from the chassis in which the power supply sits. In this case, high-voltage gas-discharge tubes can be used in series with the MOVs to ensure the needed insulation.
Another consideration is to protect the output on the secondary side of the power supply to minimize the electrical stress placed on the circuitry behind the power supply. This is done in cases when it is known that the circuit is susceptible to transient voltages. In this case, a TVS diode (leaded or surface mount) is installed on the dc output lines of the power supply.