The application range of pulse transformers is very broad. In most cases, a signal or a control pulse must be transmitted between electrically isolated circuits. This problem exists in the activation of thyristors and triacs, or in the operation of FETs or IGBTs in highpower switching circuits. Another application involves electrical isolation in telephone switchboards and data transfer systems.
When used in power electronics, the secondary side of pulse transformers is normally at a high voltage potential. This requires a high insulation strength for pulse transformers.
Complying with VDE 110 b, Part 1, the following test voltages between the primary and the secondary circuits are required for transformers of protection class I and choke coils, as a function of the working voltage:
Working voltage [V]
Test voltage Uisol [V]
The test voltage for SCHURTER pulse transformers depend on the type of winding and coating on the coil wire. Exact information concerning each type is available in the technical specifications. The test voltage is in each case considerably higher than that prescribed by VDE 110 b.
Partial discharges during normal operation have little effect on the operation of the circuit, but can accelerate the ageing of the pulse transformer. The glow discharge and the intermittent voltages are at least 50% higher than the approved working voltages for all SCHURTER pulse transformers. This provides the best assurance against long-term damage.
Over the almost straight-line in the lower 2/3 of the rise curve, i.e. in the area where the semiconductor is triggered with certainty, we draw a line and measure the time from 10% to 90% of the overall pulse height.
The measurement is made with the following circuit. The load resistance RL is given for each type.
For a turn ratio of 1:1, the test voltage is 10V; For a turn ratio of 2:1, the test voltage is 20V, and so on.
The maximum trigger current is a guide value. For a given current, the drop in voltage over the secondary winding resistance is smaller than one volt.
The voltage-time integral is the product of the pulse height and width, measured at half pulse height. The voltage-time area is measured on the secondary side during operation under no load.
The voltage-time integral Us • Tw is measured according to the principle of the following circuit. The same voltages as used for measuring the rise time are used.
Primary and secondary inductances are measured with a low-power signal of 0.1 mA/10 kHz at 25°C. The tolerance is -30% / +50%. The measured value can also vary up to ± 25% under temperature variation in the range 0°C to 70°C.
The coupling capacity is measured between the primary and one secondary winding. This value varies depending on the type of winding. Bifilar windings, designed for models with faster rise times, have higher coupling capacitances than the layer or selection windings.
In general, this value is not important with regards to transmission properties. To guarantee effective interference protection from the control electronics, however, the smallest possible coupling capacity is desired.
In the given turn ratios, the first figure always refers to the primary winding. Hence a «1:1» pulse transformer has the same number of winding on both the primary and the secondary windings. The turn ratio «3:1:1» stands for one primary and two secondary windings with a transformation ratio of three to one between the primary and the secondary windings.
SCHURTER offers pulse transformers with other turn ratios than specified on the data sheets upon request.
Example of application:
Power transistor in pulse operation
The plastic cases and the potting resin of all SCHURTER pulse transformers are fire resistant in compliance with UL 94 V-0.
Abbreviations used in the technical data:
Voltage-time integral (Us•Tw)
Pulse rise time
Power dissipation at ambient 50°C
Power dissipation at elevated temperature
Test load resistance (secondary)
Primary inductance = Ls x N2
Working voltage primary-secondary in VRMS