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.
a) Pulse Width Modulation Devices PWM Devices
A very common application is PWM, which is employed in various automotive, industrial, and alternative energy applications, including LED drivers for luminosity control; frequency inverters that match the output voltage and frequency to 3-phase AC drives; and DC motor drivers for speed adjustment.
A DC motor driver circuit is illustrated in Fig. 1. Here, PWM is realized by frequent switching of four MOSFETs arranged in an H-bridge. The motor voltage is then determined by the switching frequency as well as the switching speed, which is controlled by the gate resistors. Although quick switching is favoured because of low switching losses, it can induce undesired EMI signals, or ringing of the circuit. As the gate resistor is exposed to the PWM pulses, it must have a sufficient pulse load capability.
b) Incidental Pulse Loads
Pulse-load-sensitive elements in electronic circuits can be protected by the integration of dedicated sub-circuits. These typically work as bypass or load sinks and feature different components ranging from smoothing capacitors to voltage-suppressing diodes. Their common element, however, is the pulse-resistant resistor, inserted to absorb the pulse power or lower the pulse voltage over the pulse-sensitive component to a non-critical level. Sources of incidental pulses are grid- or field-bound EMI signals, including broadband burst pulses induced by switching actions, surge pulses caused by lightning, as well as ESD.
Protection against burst pulses or ringing effects caused by field-bound EMI signals in electrical systems with inductive or capacitive loads is provided by RC-snubber circuits. These have an attenuating effect on the voltage rise across the switching device and dissipate the energy of the resonant circuit.
Nearly every electronic application is potentially exposed directly or indirectly to lightning strikes that result in a surge pulse load on the electronic circuit. Surge pulses have high energy and can reach voltages of several thousand volts. Protection of sensitive components against them can be realized using the circuit shown in Fig. 2. For this, a transient voltage suppression (TVS) diode is employed to divert the surge pulse load from the application circuit and balance its voltage at an acceptable level. Here, the function of the resistor is to regulate the diode current.
The high-pulse-load resistors used in the above applications have to be selected thoroughly. For this, general application requirements — such as the resistor’s tolerance and temperature coefficient — have to be considered. Finally, the key factor is its pulse load capability, which has to be sufficient for the highest anticipated pulse load.
Because of this complexity, we will give guidance on the selection of suitable resistors by estimating the pulse load for two examples:
- A gate resistor in a motor driver (Fig. 1)
- A protective resistor in a surge pulse protection circuit (Fig. 2)
Before we turn to these examples in Section 5, we will address the factors determining the pulse load capabilities of resistors of different technologies.
