ODEP - Output Device Emulation Protection
ODEP is a thermal management system which safely allows an amplifier to produce more power than an equivalent amplifier with conventional VI limiting.
The most critical components in a power amplifier are the transistors in the output stage that drive the high voltage and current to the load. ODEP is a form of protection that ensures long term safe operation of these key transistors. In a power amplifier the output stage is the major source of heat inside the chassis, and heat is the real concern during operation.
The heat is generated at the heart of the output transistors’ chip, in what is called the die. The die is the junction area of the actual semiconductor materials. The heat generated at the die is transferred directly to the metal case of the transistors; then from case to heat sink; then from heat sink to the atmosphere. Figure 1 shows an “exploded view” of an output transistor device on a heat sink. It also shows the electrical equivalent to both steady-state (DC Model) and dynamic (AC Model) conditions.
Heat vs. Time
An amplifier’s power rating is typically based on its maximum continuous output.
With that in mind, remember that the heat produced in the output device must transfer across the die to case boundary, through the case, through the case to heat sink boundary, and dissipate into and finally out of the heat sink.
If the transistor sees a continuous waveform, it would eventually reach a steady state condition where the thermal sensor on the heat sink would see a temperature representing the actual transistor temperature. But often the waveforms to be amplified or the load the amplifier is driving is not constant. This will cause instantaneous heating conditions in the amplifier that a traditional sensor cannot respond to.
By the same token, once a heat sink is warmed up, the transistor cannot handle as much power because its ambient condition is already close to its thermal limits. The reduction in power handling capability per degree rise in temperature is called thermal derating. Since the thermal switch cannot detect the dynamics of non-steady-state test signals and its effect on the actual transistors (the heat transfer process takes a long time), it cannot effectively respond to dangerous thermal conditions.
Conventional Thermal Protection
As for thermal protection, most other manufacturers use a thermal sensor mounted on the heat sinks. When the heat sink gets hot enough, the sensor causes amplifier shutdown. The effect, to the user, is potentially catastrophic: complete loss of amplifier output. As described above, the basic thermal switch is also less effective as a means of protection.
AE Techron’s Thermal Protection
ODEP is, essentially, an analog computer which calculates the temperature at the die of the transistor. It senses output voltage and current, combining these to calculate output power. Heat sink temperature is also monitored. Temperature information is factored in for determining the amount of thermal de-rating. Analog circuits measure the rate of change of temperature. Power and thermal information are used to build a model of the heat transfer process, store of history of operation, and ultimately determine the actual temperature at the die of the devices. Figure 2 shows a simplified block diagram of the ODEP system.
If the ODEP circuitry determines that the test signal is about to cause a dangerous thermal condition in the amplifier, it will limit the amplifier output enough to prevent damage. ODEP does not shut down the amplifier, although if conditions are extremely severe it can almost completely limit the amplifier output to prevent damage. With ODEP, the amplifier is protected at all times.*
Crown International (AE Techron’s former parent company) holds the patent on ODEP, and AE Techron is the only industrial amplifier manufacturer to use this type of circuitry to calculate real thermal conditions.
*If your test conditions do not permit a change in amplifier output, the AE Techron 7000 amplifiers can be configured to go to standby should an momentary ODEP event occur.