Posted on 23 April 2019

Criteria for Successful Selection of Diodes and Thyristors








Thyristor and Rectifier Dimensioning and Selection

The general guidelines given in the article "Criteria for Successful Selection of IGBT and MOSFET Modules" are to be applied by analogy to the selection of thyristors, rectifier diodes and thyristor/diode modules. For operation at a line frequency of 50/60 Hz, any additional stresses caused by switching losses may be neglected. When selecting power semiconductors for actual applications, the following aspects have to be taken into account:

- Voltage load capacity,

- Current load capacity under the achievable cooling conditions

- Permissible operating areas.

The aforementioned aspects must be factored into considerations for all stationary and short-time operating conditions (overload). The same applies to the limits specified for the following: vibration and shock resistance, resistance to extreme climatic conditions, insulation voltage in modules, and assembly and mounting instructions. An exception with regard to the reverse voltage is avalanche-resistant diodes for a maximum avalanche power dissipation PRSM; an exception relating to the junction temperature is a surge current event.

To achieve a high degree of reliability and sufficient service life, the module capacity must factor in the intended number of load cycles at which notable temperature cycles occur. Furthermore, "serious" dimensioning is generally not based on the thermal capacity of the semiconductors up to the limit value Tj(max) in order to leave a safety margin for cases that have not been considered in theory and for module aging.

1) Reverse voltage

In diodes and thyristors, the on-state losses are less dependent on the voltage rating than is true for MOSFETs and IGBTs. Therefore, when selecting the component voltage class, the safety margin between the component reverse voltages that usually occur and the permissible reverse voltages must be sufficiently large. Normally, the following reverse voltages are selected for diodes and thyristors for line voltage VN.

Figure 1. Recommmended reverse voltage for thyristors and rectifier diodes dependending on the rated current of the line

Ensure that at the maximum voltage load, the maximum permissible module voltage, is not exceeded. This applies in particular to stationary input voltage (rated voltage + tolerance, e.g. +10%) and transient overvoltage, provided these are not reduced by line filters, DC link capacitors and dc-side protective devices (suppressor diodes, snubbers, varistors). For transient voltage peaks, a slightly higher peak voltage (VRSM) is often permissible. Note that the reverse voltage specified for 25°C is temperature-dependent and has a positive temperature coefficient. This value is dependent on the reverse voltage of the component itself and can amount to a few V/K.

2) Rectifier diodes

 Thermal load in continuous duty

The forward current load in continuous duty is related to the product of the average power dissipation PFAV and total thermal resistance Rth(j-a) of the device.This product must not exceed the difference between the ambient temperature Ta and the maximum permissible virtual junction temperature Tj:

\begin{equation} P_{FAV}\cdot R_{th(j-a)}\leq T_j-T_a\end{equation}

The following general formula is used to calculate the power dissipation:

\begin{equation} P_{FAV}=V_{FO}(T_j)\cdot I_{FAV}+r_F(T_j)\cdot I_{FRMS}^2\end{equation}

where rF is the forward "slope resistance" (found in product data sheets).

For the typical diode current waveforms (180° sinusoidal, 120° rectangular), the current (wave) form factor can easily be calculated as follows:

\begin{equation} F_I=\frac{I_{FRMS}}{I_{FAV}}\end{equation}

Thus, PFAV  still depends on only one unknown; in most cases, representation with I FAV is selected:

\begin{equation} P_{FAV}=V_{FO}(T_j)\cdot I_{FAV}+r_F(T_j)\cdot F_I^2\cdot I_{FAV}^2\end{equation}

(Note: In the equations above, FAV=forward average; FRMS=forward root mean square; FO=forward on-state)

These formulae form the basis of almost every component design software tool. Nonetheless, datasheets contain numerous graphs and characteristics to help the user with product selection.

3) Thyristors

Load in continuous  duty

In thyristors the pulsing of the virtual junction temperature in line with the operating frequency has to be taken into account when determining the limiting value of the mean forward current. In thyristors that change the conduction angle in order to control the output voltage, the result is rectangular pulses with shorter conduction angles under inductive load or "controlled" half sine waves for resistive load.The difference between thyristors and rectifier diodes is that in the case of thyristors it is not the heat sink thermal resistances Rth(c-a) , but the thermal resistances R th(j-a)  of thyristor and heat sink together that are indicated over the ambient temperature Ta .

Critical rate of rise of current and voltage

In addition to pure thermal rating, for thyristors, care must be taken that the critical rate of rise of current (di/dt)cr and the critical rate of rise of voltage (dv/dt)cr  are adhered to. The RC element that is normally connected in parallel to the thyristor already induces a considerable di/dt. This is why the rate of rise of current caused by the rest of the circuit should lie far below the critical value. In rectifiers, even relatively small RC elements can result in a reduction of the voltage rate of rise to very low values that the thyristors can easily cope with. The same applies to a.c. power converters and circuit breakers. In inverters, and especially in 4-quadrant converters, by way of contrast, the rapid drop in voltage when the thyristor is triggered causes steep voltage peeks at the thyristors of the other branches in a forward direction. These voltage peaks are barely attenuated by the RC circuit since the only series resistance is the switching inductance. This is why, for such applications, thyristors with a high critical voltage rate of rise (1000 V/μs) are used.

4) Bridge Rectifiers

Bridge rectifiers are components which have every branch of a rectifier circuit in a single, compact case. Bridge rectifiers exist for use with currents ranging from a few amps to several hundred amps in different case types. With a few exceptions, the heat losses are dissipated via a base plate or the DBC to a cooling plate or a heat sink.


For more information, please read:

Snubber Circuits Based on Silicon Avalanche Diodes

Safe Firing of Thyristors

Reliable Thyristor Triggering

Driver Units for Thyristors

Parallel and Series Connections of Power Semiconductor Devices


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