Selecting Surface Mount Bridge Rectifiers

A bridge rectifier converts alternating current into direct current with four diodes in a single package. The surface mount versions are compact, inexpensive and available in a wide range of ratings, and the differences between them matter more than the headline current figure suggests. Choosing a part on current alone is a common way to end up with one that runs hot or that fails at inrush.

The parameters that decide whether a part is suitable are the reverse voltage, the forward current and its derating, the forward voltage drop, the surge current capability and the thermal resistance from junction to the mounting surface. Each of them interacts with the others, and the package size links them all.

Reverse Voltage and Its Margin

The reverse voltage rating has to exceed the peak reverse voltage the part sees in the circuit, which is not the same as the root mean square input. For a bridge rectifier fed from a sinusoidal supply and loaded with a capacitor, the peak reverse voltage across each diode is approximately the peak input voltage, and it rises further when the supply is at its high line limit and the load is light. A margin of at least twenty percent over the worst case peak is a reasonable starting point.

Transients are the other consideration. A switching event or a line surge can push the voltage well above the steady state peak, and a part with a rating just above the operating point will avalanche. The avalanche energy it can absorb without damage is specified for some parts and not for others, and where the circuit has a known transient the specification should be checked. Adding a varistor or a transient suppressor at the input is often cheaper than specifying a higher voltage bridge.

Current Rating and Derating

The current rating of a bridge rectifier is specified at a case temperature, and it falls as the temperature rises. A part rated at two amperes at twenty five degrees may be rated at one ampere at a hundred degrees, and a design that uses the headline figure without derating will run the junction above its limit. The derating curve in the datasheet is the parameter to use, and it should be applied with the actual case temperature rather than the ambient.

The current waveform also matters. A bridge rectifier feeding a capacitor draws current in short pulses near the peak of the input waveform rather than continuously, so the root mean square current in the diodes is higher than the average output current and the conduction loss is concentrated in those pulses. Calculating the loss from the average current alone underestimates the heating by a significant factor, which is why a rectifier often runs hotter than the output current would suggest.

Forward Voltage and Loss

Two diodes conduct in series in a bridge, so the forward drop is added twice in the current path. A part with a low forward voltage reduces the loss and the heat, and the difference between a standard junction and a low drop type is worth a look in a design where the efficiency matters. Schottky diodes have a lower forward drop but a lower reverse voltage rating and a higher reverse leakage, so they are used in low voltage supplies rather than in mains rectification.

The forward voltage also varies with current and with temperature. The datasheet curve shows the drop falling as the temperature rises for a silicon diode, which reduces the loss and partially offsets the thermal effect. For a loss calculation the drop should be taken at the expected junction temperature rather than at room temperature, and the calculation should be iterated because the junction temperature depends on the loss.

Surge Current and Inrush

When the supply is switched on with the capacitor discharged, the rectifier sees a surge current limited only by the source impedance and the diode resistance. The surge rating of the bridge is specified as a peak current for a defined duration and a defined number of cycles, and it is usually far higher than the continuous rating. A part that is adequate for the steady state current can still be damaged by the inrush if the capacitance is large or the source impedance is low.

Managing the inrush is a circuit problem rather than a component problem. A negative temperature coefficient thermistor in series limits the initial current and then falls in resistance as it heats, and a resistor with a relay that bypasses it after startup is another option. Where the inrush is unavoidable, the bridge should be selected from the surge rating rather than the continuous one, and the surge rating should be checked against the worst case, which is a switch on at the peak of the mains waveform.

Thermal Design and Package Choice

The thermal resistance from junction to the mounting surface determines how much heat has to leave through the board. A small package has a higher thermal resistance and therefore a higher junction temperature for the same loss, and the copper area connected to its pins is part of the thermal path. The datasheet thermal resistance is usually specified with a defined copper area, and a design that uses less copper will run hotter than the figure implies.

The package also determines the creepage and clearance that can be achieved, which matters in a mains application. A small package has its pins close together, so the isolation distance has to come from the layout and from any slot cut in the board. Where the supply has to meet a reinforced insulation requirement, the bridge package and the layout have to be considered together, and the EMI suppression design and the safety distances should be reviewed at the same time rather than separately.

Putting the Selection Together

The selection sequence is to establish the peak reverse voltage including margin, calculate the root mean square current in the diodes from the actual conduction waveform, choose a package whose derating curve supports that current at the expected case temperature, and then check the surge rating against the inrush. If the junction temperature calculation comes out too high, the options are a larger package, more copper under the part, a lower forward voltage type or a change to the circuit that reduces the inrush.

Each of these decisions should be recorded with the calculation, because the assumptions are the part that is most likely to change later. A different capacitor value changes the inrush, a higher ambient changes the derating, and a change of supplier may change the thermal resistance. Keeping the calculation together with the acceptance results makes the substitution straightforward and prevents a part being replaced by one that meets the headline rating but not the conditions the design actually imposes.

Additional Considerations for This Build

Practical attention to current derating pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating current derating explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can a higher current bridge always replace a lower one? Usually, provided the package fits and the reverse voltage rating is adequate. The higher rated part often has a larger die and a lower forward voltage, which reduces the loss.

Why does the bridge run hotter than expected? Because the current is drawn in pulses rather than continuously, and because the thermal resistance is specified with more copper than the layout provides.

Is a Schottky bridge suitable for mains input? No. Schottky diodes have a low reverse voltage rating. They are used in low voltage secondary side rectification instead.