Other Parts Discussed in Thread: LM5180-Q1, LM5180
By Jiri Panacek, Systems Engineering and Marketing, Freising, Germany
How can my 48-V automotive system design pass the E48-02 transient overvoltage test to meet VDA 320?
Today’s 48-V automotive systems, such as those used in mild-hybrid electric vehicles (mHEV), must typically comply with the VDA 320 (LV 148) test specification. This specification is a base for the recently published ISO 21780:2020 standard. The norm specifies various test conditions for loads connected to the 48-V net.
The E48-02 transient overvoltage test presents an interesting challenge; the voltage on the 48-V rail must remain at 70 V for 40 ms (some OEMs even require 100 ms). The device under test (DUT) must survive this event with functional status A while performing all functions. Additionally, VDA 320 specifies a long-term overvoltage test that requires the 48-V rail to remain at 60 V for 60 minutes. Unlike 12-V automotive battery systems, there is no specified load dump requirement (related to an alternator's inductive behavior) in a 48-V battery system.
ICs connected to the 48-V rail must withstand 70 V at all times
Clamping the voltage rail using a transient voltage suppressor (TVS) or Zener diodes is impractical or impossible for higher loads due to excessive power dissipation. Simply said, power converters connected directly to the 48-V rail must withstand 70 V under all conditions.
Integrated circuits (ICs) use various technological processes that are optimized for the intended application. Increasing the input voltage range typically results in higher cost due to a more expensive manufacturing process. Do we really need to have 85- or 100-V rated designs for 48-V systems?
Not always, the LM5180-Q1 and LM5181-Q1 devices are primary-side regulated (PSR) flyback converters with an input voltage range from 4.5 V to 65 V and an absolute maximum input voltage rating of 70 V. The control scheme samples the output voltage directly from the primary side. This eliminates the need for an optocoupler or transformer winding for output voltage feedback. The characteristics and easy implementation of the LM5180-Q1 and LM5181-Q1 make these converters an attractive choice for 48-V automotive systems. However, the input voltage range does not satisfy the transient overvoltage test scenario. Is there an easy solution?
Yes, you can pass the transient overvoltage test with the LM5180-Q1 and LM5181-Q1.
This design tip presents a simple discrete linear regulator (LDO) that passes all voltages below approximately 63 V but acts as a drop-out regulator for voltages above this threshold.
The circuit in Figure 1 using the LM5180-Q1 operates in three phases with respect to the input voltage:
Start-up phase – Current from the 48-V rail flows through the resistor R1 to the base of the transistor Q1. Q1 starts conducting and current from the 48-V rail charges the input capacitor. This phase ends when the voltage on the input capacitor exceeds the undervoltage lockout (UVLO) rising threshold and the LM5180-Q1 starts switching.
Pass-through phase – The LM5180-Q1 is switching normally. The diode D1 rectifies the pulsating voltage from the switch node (SW) and charges the capacitor C1. The voltage on C1 corresponds to the reflected voltage reduced by the voltage drop across the diode D1. The voltage on the capacitor C1 is positive with reference to the emitter of the transistor Q1. The resistor R2 further biases the base of the transistor Q1, and the bipolar transistor enters saturation. The collector-emitter voltage drop VCE is VCE (SAT) and transistor Q1 operates as a switch.
Pre-regulator phase – The Zener diode ZD starts sinking current when the input voltage exceeds the Zener voltage VZ. The voltage on the output of the pre-regulator is limited to the Zener voltage raised by the base-emitter voltage drop.
Figure 1: Pre-regulator circuit protecting the LM5180-Q1 PSR flyback converter (including the switch node).
Additionally, the circuit can be modified for use in other type of converters. Any kind of switching can be repurposed for a simple charge-pump that delivers enough current to the base of the transistor Q1.
The circuit in Figure 1 has reduced thermal stability due to the temperature drift of the Zener diode ZD and base-emitter voltage.
For complete guidance on designing this circuit and improving its thermal stability with -431 shunt regulators, see the application report, “Extending the Input Voltage Range of the LM5180-Q1 PSR Flyback for VDA 320 (LV 148).” This application note provides the means by which the LM5180/1-Q1 may be cost effectively used in a 48-V environment and meet the published spec with some margin.