How can I not burn the RF PA？

PA (Power Amplifier) is an important component of the communication system, which is responsible for amplifying and powering RF signals. The performance and reliability of PA have a significant impact on the entire transmission system.

In the mobile phone communication system, the output signal of PA is about 29~32dBm (about 1,000mW), which is 1000 times larger than the power output of about 0dBm (1mW) of the Transceiver chip. At the same time, in order to achieve the corresponding RF power output, the PA generally needs to consume about 1,900-4,000mW of DC power consumption.

Figure 1: Typical mobile phone communication system link

High-power, high-power applications also pose challenges to PA reliability. In system applications, "burning PA" is a hot topic in the discussion of engineers.

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Why do you burn PA

Semiconductor devices in mobile phone PA chips

The most commonly used semiconductor process for PA design is GaAs HBT,GaAs pHEMT,SOI CMOS,Bulk CMOS (bulk silicon CMOS). Among them, GaAs HBT has become the preferred process for the power output stage of RF power amplifiers due to its high power density and low cost (compared to GaAs pHEMT).

HBT devices have three limit parameters, namely:

maximum allowable current of collector Icmax maximum allowable thermal dissipation power collector-emitter reverse breakdown voltage BVCEO In practical work,

it is necessary to ensure that the maximum working circuit, dissipated power

and maximum voltage of the device are within the rated range.

The voltage/current swing of the PA

For HBT devices with RF PAs, the collector of the output transistor has a maximum voltage and current swing. In order to better characterize the relationship between voltage and current, the voltage and current are usually plotted in the same diagram in the form of a load line, as shown in Figure 2.

The load line of the transistor reflects the relationship between the voltage and current of the transistor under different loads, and is generally drawn on the DC-IV curve. In the load line:

The slope of the load line reflects the magnitude of the load impedance.

In the actual circuit, due to the existence of the load imaginary part, the voltage/current phase difference will be formed, which may cause the load line to appear as a hollow ring.

3. The selection of DC operating point (voltage and current) has an impact on the swing of the load line.

Figure 2: A typical dynamic load line for a PA

In PA operation, due to the existence of high power supply voltage and high VSWR, the PA output voltage and current swing will increase. Figure 3 shows a typical power amplifier with dynamic load lines at 50Ω and VSWR=10:1 at 3.2V and 5V bias voltages [2]. It can be seen that under high voltage and VSWR, the PA will need to withstand a larger voltage and current swing. When the voltage and current swings exceed the tolerance value of the device, the device will burn out.

Figure 3: Changes in PA dynamic load lines at different voltages and VSWRs

How to protect the PA from burning

Design Guarantees

PAs need to be properly designed to meet the needs of Ruggedness.

Current design guarantee

It is necessary to design the size of the device reasonably to ensure that the maximum current through the device under various conditions is less than the maximum withstand current of the device. In the design of the through-current, it is important to note that multiple transistor devices are connected in parallel at the end stage of the PA, and it is necessary to ensure that the current is evenly distributed in the whole device, rather than all the current is concentrated in one device and the device is burned. Since the turn-on voltage of the HBT device decreases with increasing temperature, excessive current will reduce the turn-on voltage and increase the current further until the device burns out.

This effect is called Thermal Run-away and is a common form of current burning. In order to prevent the occurrence of thermal run-way, a Ballast resistor is added to the base or emitter side of the transistor. The presence of the Ballast resistor causes the Vbe voltage to decrease when the current increases, preventing the current from increasing further.

Figure 4: Uneven thermal distribution of PA (left) and design of Ballast resistors

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Voltage design guarantee
For voltage protection, the voltage is generally regulated by placing diode strings in parallel in the final transistor collector so that the output swing is stable at the opening voltage of the diode string. In the design of voltage protection circuits, it is necessary to ensure the symmetry of the placement of the protection circuits to ensure that the voltage swing of all devices is protected. Ruggedness Test Since PA reliability is difficult to accurately design by simulation, after the PA design is completed, it must pass a complete Ruggedness test to ensure the reliability of PA. The complete Ruggedness test environment is shown in the figure below. The Ruggedness test needs to cover the following test conditions:

Figure 5: Ruggedness test environment

The above test items need to be cross-combined to ensure that there will be no ruggedness problem in the PA under any conditions. Since the maximum breakdown voltage of semiconductor devices decreases with decreasing temperature, and the PA gain increases with decreasing temperature, the worst point of ruggedness generally occurs at low temperatures. Therefore, at low temperatures, the maximum input power, the highest voltage, and the maximum VSWR are the worst conditions for Ruggedness.

App Assurance

Although qualified PAs are fully ruggedness tested before leaving the factory, it is still necessary to pay attention to the application environment to ensure that the Ruggedness is guaranteed in the application. The main safeguards required in the application are as follows:

Control the power supply voltage appropriately
As shown in Figure 3, PA has a smaller voltage and current swing in low voltage applications, and the ruggedness of PA will be better guaranteed. Therefore, in the application, properly controlling the voltage and using the supply voltage as low as possible can help to improve the ruggedness of the device.

Properly control the output power
When the power output is high, the PA output will have a greater voltage and current swing. Adapting to the control of output power within the allowable range of the application will help to improve the Ruggedness.

Pay attention to power integrity and signal timing
The mobile phone is a rather complex system that involves the linkage between multiple modules. In the application, it is important to pay attention to the integrity of the power supply (whether there are excessive voltage pulses), the timing of the bias control signal, the size and timing of the input signal, and the timing to ensure that the PA is operating in a normal state.