RTP Protection of Power Control PCBs. Neal Schultz

Transcription

1 RTP Protection of Power Control PCBs Neal Schultz

2 Introduction Thermo mechanical stresses, harsh environments, or multiple short circuits might lead semiconductor devices to fail. Semiconductor failure mechanisms are unpredictable, they can be open circuit, short circuit, or resistive. Open circuit failures are typically benign in the sense they leave no residual or unintended power flowing thereby negating the need for additional protection circuitry. Short circuits are typically dealt with through the installation of current fuses which will lead to the open circuit condition above. Resistive failures are probably the hardest to deal with because they can cause overheating while the circuit electrical parameters remain within normal operating conditions.

3 Introduction Continued In some cases, just a few watts of unintended power dissipation can drive local hot spot temperatures several hundred degrees centigrade above ambient and lead to a printed circuit board failure. Standard printed circuit boards have a glass temperature of 135 C, above this temperature the PCB epoxy structure is impacted, and micro short circuits may appear leading to catastrophic thermal events. Thermal protection may be used to mitigate these hazards. To that end TE Connectivity has introduced a family of Reflowable Thermal Protectors (RTPs).

4 Components Failure Mode Failure mode vs Current Constant Low Current High Power Low Power No Power Short Circuit Resistive Circuit Open Circuit

5 Failure mode vs Current Constant Low Current High Power In that case, thermal runaway risks! Low Power No Power Short Circuit Resistive Circuit Open Circuit

6 Power Component Resistive Failure Mode A resistive short may occur leading to an unsafe thermal event Resistive shorts can produce unsafe temperatures due to I 2 R heating As little as 10W may generate a localized hot spot of more than 180 C, potentially damaging the board s epoxy structure Not enough current to blow the upstream fuse

7 Application Example:Failed Power FET, Thermal Shutdown During Assembly: RTP device does not open in 260 C SMD reflow process In service RTP opens if FET failure causes dangerous Over Temperature conditions RTP opens before solder melt point: 220 C Implementation Schematic RTP Car Battery P 1 RTP P TH FET Power FET Control IC (May be ignition controlled) Load Resistance Solution Considerations FET failure More Current Current may even decrease if FET fails Does not open if FET is hot (i.e. 175 C junction) but operating normally Survive 260 C reflow, Open before 220 C if FET fails

8 Reflowable Thermal Protector (RTP) Operating Principle Path for the power to go through; Thermal Protector opens if T> T open RTP is a 3 Pin Device P 1 : Power pin P TH : Power and Thermal Sensing From: Power source (Battery) P 1 Thermal Protector Arming Fuse ARM P TH RTP To: Power component ARM : Electronic Arming Pin ARM P 1 P TH Path for arming procedure; once armed the ARM pin will not be used anymore Pre- Arming Device will NOT Open in multiple reflows Arming Arming the RTP is a ONE TIME event Post - Arming Device will open when temp at P TH exceeds T open

9 RTP Pre and Post Arming Procedure No link open Arming link is open Thermal spring is open AFTER REFLOW The device can survive up to three 260 C reflow passes without opening ARMED Arming timing is user-determined and can occur during validation test phase OPEN The device will open after the critical junction temperature exceeds the Open temperature

10 RTP200R060SA Key Characteristics Surface mount 0.6m 200 C open temp 200A max 16Vdc

11 Block Diagram Cooling Fan Module 50A Fuse EMI FILLTER RTP device PWM Controller Module 12 or 24 Vdc Vbat Input Speed DC Motor p Battery Side Load Side

12 HVAC CFM 3 phase BLDC Sensitive area 50A Fuse RTP DC bus 12 or 24 Vdc Vbat DRIVER DRIVER DRIVER BLDC Motor p Speed input PWM Controller Shunt Resistor Last defense Protection in abnormal condition ( thermal runaway from Power FET or capacitor failure )

13 HVAC CFM Brush DC Motor Sensitive area 50A Fuse Thermal Fuse DRIVER 12 or 24 Vdc Vbat Speed input PWM Controller DC Motor p Shunt Resistor Last defense Protection in abnormal condition ( thermal runaway from Power FET or capacitor failure )

14 FET s can Fail in the Field Even when in Specified Operating Conditions Good FETs can go Bad FET s can pass initial testing & fail later Random weak points will result in field failure given correct conditions Image taken from IR paper showing random locations of failure points for devices tested to failure FETs fail vs time and exposure Customers report generating failures without exceeding FET operating limits. (Given harsh operation & time) We can generate failures with OV +50V exposure Failure Rates (PPM) Year 1 Failures Vs Time Condition 1 Condition 2 Year 2 Year 3 Year 4 Year 5 Year 6 Failure mode: Random Resistive Shorts Customers report that resistance varies Customers that failure resistance is random and unpredictable Resistance (Ohms) Failed Device Resistance Device #

15 Failure analysis: Power FET Failure in Resistive Mode Not Enough current to blow the Fuse Fuse 70A Low Current CS RTP FET fails in resistive Mode FS N-Channel Power FET D S Battery FET G Load Resistance Control IC (May be ignition controlled)

16 PCB layout setup Heatsink hot spot Huge internal Heatsink Heatsink RTP FET Top layer Layer 2 Other layers Layers n 3,4 4 layers: 70µ (top) + 105µ +105µ +70µ (bottom) PCB material: FR4 Rth~13 C/W page 16 /

17 Power FET failure in resistive Mode 10A 9A-9.3W 5.6W 8A-7.8W Rth~13 C/W- Ambient Temp = 25 C Beyond 10W, the FET goes into a slow thermal runaway. The RTP pad temperature tracks the Power FET temp until the RTP device opens the circuit interrupting current flow. page 17 /

18 D2pack Power FET failure in resistive Mode 10A Power FET :200 C (AVG) RTP pad 160 C (Avg)

19 IMS Insulated Metallic substrate Insulated Metal Circuits (IMCs), sometimes called Insulated Metal substrates (IMS), are thermally conductive circuit boards that provide heat sinking and heat spreading ability and therefore allow designers to implement high watt density designs with confidence. Thermal performance on this kind of support allows very good heat transfer from the potential heat source (Failed Power FET) to the thermal protection component (RTP device), even if the distance between components is beyond 10cm.

20 Power FET Small thermal gradient RTP Thermal response on IMS with 10cm distance between Power FET and RTP device. Thermal gradient between both devices is less than 40 C over 10cm length!

21 Applications HVAC blower, engine cooling Fan module ABS EPS Start / Stop Glow Plug PTC Heater DC/DC converter Other

22 Thank you for your attention! Questions?