Vishay Intertechnology application note on the resistor pulse load characterisation. The article compare different resistor manufacturing technologies from the point of their capability to handle high power pulse load. Carbon film MELF has been identified as the champion – you can read more about the reasons and technology backgrounds.
INTRODUCTION
Many electronic circuits are exposed to high pulse loads. In some applications these occur regularly, such as in pulse width modulated (PWM) devices. In others pulses are incidental, but also inevitable — resulting from electromagnetic interference signals (EMI). Unfortunately, due to the increasing miniaturization or intrinsic limitations of electronic components, their pulse load capability is often insufficient to withstand these pulse loads and they require protection.
Whether pulse loads are regular or incidental, pulse-proof resistors are needed, and MELF resistors are especially well suited to this application due to their excellent pulse load capability — a prominent feature linked to their unique cylindrical design. In addition to standard metal film technology, MELF resistors are available with carbon film, which further enhances their pulse load capability. The largest case size (0207) carbon film MELF not only provides the highest pulse load capability of MELF resistors, but of all SMD film resistors, thus making it the optimum choice for high-pulse-load applications.
This article will help designers in selecting the optimum resistor for protection by identifying the pulse properties — power, duration, and shape — that need to be considered. For this, the effects of pulses on resistors and the factors determining their pulse load capability are explored, and the resulting advantages of carbon film MELF resistors are illustrated.
Finally, we will illustrate the selection of suitable resistors for high-pulse-load applications using the two examples presented in Section 2: the gate driver circuit in a motor driver application and the surge pulse protection.
a) PWM Devices: Gate Driver Circuit
In this example we will deal with the motor driver shown in Fig. 1. The motor speed is adjusted by PWM, realized by the frequent switching of MOSFETs. To control the MOSFETs’ switching speed, a gate resistor is used. Since the resistor is likewise exposed to pulses, it must feature a high pulse load capability.
Although different gate resistors are often employed to adjust on / off switching speed separately, here for simplicity we consider just one (RG).
Switching of the MOSFET requires recharging of its gate capacitance with a gate charge QG, which is achieved by the application of a driving voltage UD. This process, which follows an exponential behavior described by the time constant t, determines the switching speed. It is biased by the (dis)charging current i = dQG / dt, which in turn is controlled by the gate resistor. Thus, the gate resistance is defined by the desired switching duration. In our example we consider a motor that runs at a frequency of f = 17 kHz. A MOSFET of gate charge QG = 2 μC is chosen, and driven by UD = 20 V. The duration of the exponential switching pulse shall be t = 200 ns.
Selection of Components
- To define the switching duration, the resistance has to be set to
- The power load on the resistor has to be estimated. The PWM creates a continuous pulse train. Thus the resistor is exposed to continuous pulses as well as an average power
- The average power is determined by the energy content of the gate capacitor released in every on / off switching process, and the switching frequency:
It is equivalent to the power load on a resistor under continuous operation. In datasheets, the permissible limit is specified as the rated dissipation P70. A comparison of the P70 of the SMD resistors considered above shows that only the largest case size (0207) metal and carbon film MELF resistors, whose rated dissipation is 1 W, are able to withstand this average power.
- The power per pulse is determined by the gate capacitor’s energy content WG as well as the switching pulse length t:
The pulse load capability for continuous pulse trains is specified in the datasheet of the resistor. It can be found in the continuous pulse diagram, which is shown for the carbon film MELF resistor in Fig. 8.
In this diagram, rectangular pulses are considered. Therefore, to allow comparison, we have to convert our exponential pulse to a rectangular pulse of equivalent energy content, resulting in a change of pulse length according to tr = t / 2. Thus, in the pulse load diagram we have to compare with a pulse duration of tr = 100 ns. Verifying the specified permissible pulse load reveals that the carbon film MELF resistor fulfills all requirements.
The gate resistor used in this motor driver application to adjust the MOSFETs’ switching speed is continuously exposed to pulses. The selected resistor therefore must not only meet the pulse load requirements, but also withstand the resulting average power. Again, due to its outstanding pulse load capability, only the carbon film MELF resistor is found suitable for this application. Since its pulse load capability is much higher than necessary, it allows for MOSFETs of larger gate charge or shifting to shorter switching times.
b) Incidental Pulse Loads: Surge Pulse Protection
In this example (see Fig. 2) we consider a 24 V application with input resistance of RIN = 10 kW. The application is also assumed to be tolerant against voltage variations of up to 38 V. However, it has to be protected against any surge pulse of a larger voltage. Here this is achieved by diverting the pulse load via a TVS diode. For our estimates, we assume that we want to protect the application against a surge pulse of .
To handle board space limitations, the TVS diode should be as small as possible. This, however, limits its peak pulse power capability. Therefor a protective resistor is required to limit the diode current. These components have to be selected such that the input voltage of the application circuit is limited to UO = 38 V and the permissible power / voltage load on both components is not exceeded.
Selection of Components
- The resistance RP has to be balanced so that it limits the diode current appropriately, but also does not affect the application circuit under normal conditions. As it represents a voltage divider together with the input resistance, it has to be chosen much smaller, e.g. RP = 0.02 x RIN = 200 W.
- The peak diode current under surge conditions is then limited by the resistor, neglecting the much smaller diode voltage.
- The diode is selected according to the application’s specifications:
Reverse clamping voltage of about 38 V, to limit UO accordingly.
Breakthrough voltage slightly above 24 V, so that it is not conductive under normal conditions.
Peak pulse power capability larger than the maximum peak power load on the diode in this application. - Finally, the 200 W resistor must be able to withstand the surge pulse of 1 kV. This information is found in the 1.2 / 50 pulse diagram of the resistor’s datasheet. A corresponding diagram for the carbon film MELF resistor is shown in Fig. 9.
A comparison of permissible pulse voltages for 1.2 / 50 pulses for resistors of different technologies shows that only the carbon film MELF resistor exhibits a suitable limit of = 1.2 kV for this resistance value.
SUMMARY
The high peak power in pulsed and pulse protective applications requires SMD resistors of sufficient high pulse load capability. Failures of SMD resistors under pulse load are induced by the temporary strong heat generation in the device. Advanced carbon film MELF resistors have been specifically designed to combine the most important characteristics for high pulse load capability:
- The proven pulse-resistant cylindrical design, offering the largest effective resistive film area
- Helical trimming pattern, avoiding locally enhanced current densities
- The carbon film material, with its unrivaled thermal stability
In the largest case size available — the 0207 — the carbon film MELF resistor is the SMD champion in pulse load capability. Its performance is more than an order of magnitude better than comparable resistors of its class.
