onsemi MPS751G: Key Specifications and Application Circuit Design
The onsemi MPS751G is a PNP silicon epitaxial planar transistor designed primarily for high-voltage, high-speed switching applications. Its robust construction and electrical characteristics make it a versatile component in power management circuits, particularly where efficient switching and control are paramount.
Key Specifications
The MPS751G is characterized by several critical parameters that define its performance envelope. Key specifications include:
Collector-Emitter Voltage (VCEO): -40 V. This high voltage rating allows it to be used in circuits with significant voltage swings, making it suitable for offline power supplies and electronic ballasts.
Collector Current (IC): -500 mA. It can handle substantial current, enabling it to drive relays, solenoids, or small motors directly.
Power Dissipation (PD): 625 mW. This specifies the maximum power the device can dissipate without exceeding its maximum junction temperature, dictating heat sinking requirements.
DC Current Gain (hFE): Ranges from 50 to 150 at IC = -10 mA, VCE = -5 V. This gain is sufficient for effective amplification and switching.
Transition Frequency (fT): 50 MHz (Min). This high frequency indicates its capability for fast switching, which is crucial for minimizing switching losses in high-frequency inverters and pulse-width modulation (PWM) controllers.
Application Circuit Design: A Switching Example
A common application for the MPS751G is as a low-side switch. This configuration is widely used to control loads such as motors, lamps, or LEDs by connecting them between the positive supply rail and the transistor's collector.
Circuit Operation:
1. The load is connected between the positive supply (e.g., +24V) and the collector terminal.
2. The emitter is connected directly to ground.
3. A current-limiting base resistor (e.g., 1 kΩ to 10 kΩ) is connected between a microcontroller's I/O pin and the base of the transistor.
4. When the microcontroller output is set to a logic HIGH (e.g., 5V), a small base current flows, biasing the transistor into saturation and effectively closing the switch between collector and emitter. This allows current to flow from the supply, through the load, and to ground, thereby activating it.
5. When the microcontroller output is LOW (0V), the base current ceases, and the transistor turns off, opening the circuit and de-energizing the load.
Design Considerations:
Base Resistor Calculation: The base resistor (R_B) must be chosen to ensure the transistor is driven into saturation. The formula is R_B ≈ (V_GPIO - V_BE) / (I_C / h_FE_min), where V_GPIO is the microcontroller voltage (e.g., 5V), V_BE is the base-emitter voltage (approx. -0.7V for PNP), I_C is the required collector current, and h_FE_min is the minimum DC current gain.
Flyback Diode: When driving an inductive load (like a relay coil or motor), a flyback diode must be placed in reverse bias across the load. This diode provides a path for the inductive kickback current when the transistor switches off, protecting the MPS751G from voltage spikes that could exceed its VCEO rating.
Heat Sinking: For applications where the transistor operates near its maximum current or power dissipation limits, a small heat sink may be necessary to maintain a safe junction temperature.
The onsemi MPS751G is a highly reliable and efficient PNP transistor that excels in high-voltage switching scenarios. Its combination of a 40V VCEO, 500mA IC, and fast switching speed makes it an ideal, cost-effective solution for designers of power control systems, interface circuits, and motor drivers.
Keywords: PNP Transistor, High-Voltage Switching, Power Management, Application Circuit, Saturation Region
