Common Gate High Voltage Transient Blocking Unit
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Solution Overview
Problem
Conventional transient blocking units (TBUs) fail to provide optimal protection against high frequency oscillatory transients, leading to active switching and excessive electrical stress on the load and the TBU itself, particularly when used in conjunction with shunt protecting elements like gas discharge tubes or in aeronautical applications.
Innovation Solution
A modified TBU circuit with a two-terminal connection between the gates of high voltage transistors, which allows both transistors to remain in an off state during oscillatory transients, preventing active switching and reducing electrical stress by using a two-terminal passive gate-gate circuit to control the high voltage transistors independently of the TBU core.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional TBU core approach is used with high voltage transistors, then the TBU can provide high voltage handling capability, but it fails to provide optimal protection against high frequency oscillatory transients, leading to active switching and excessive electrical stress
Solution Approach 1:
A common gate connection is introduced as an intermediary element between the two high voltage transistors Q4 and Q5. This common gate acts as a mediator that couples the transistors together, ensuring they switch simultaneously and preventing the active switching problem that occurs with conventional TBU core approaches. The common gate connection directly addresses the harmful effect of excessive electrical stress during high frequency transients.
2Strength
If high voltage transistors are used directly in the TBU circuit, then the voltage handling capability is improved, but the device parameters of the high voltage transistors determine TBU parameters such as threshold current, resulting in reduced yield and increased cost
Solution Approach 1:
The TBU circuit is segmented into two functional parts: a low voltage TBU core (Q1, Q2, Q3) that sets the threshold current, and high voltage transistors (Q4, Q5) that provide voltage handling capability. This segmentation allows each part to be optimized independently - the core transistors can be precisely controlled for threshold current while the high voltage transistors only need to meet voltage ratings, improving manufacturing yield.
Solution Approach 2:
The common gate connection acts as an intermediary that decouples the threshold current setting function from the high voltage transistors. The TBU core controls the switching threshold through its own transistor parameters, while the common gate ensures the high voltage transistors switch simultaneously without their individual parameter variations affecting the threshold current.
3Speed
If the TBU core switches faster than the oscillatory transient period, then it can respond to high frequency transients, but this causes the TBU to undergo active switching instead of maintaining a low level constant current
Solution Approach 1:
The common gate connection serves as a stabilizing intermediary that prevents active switching during high frequency transients. By coupling the gates of Q4 and Q5 together, it ensures both transistors switch simultaneously and maintains their off state during oscillatory transients, even when the transient frequency exceeds the normal TBU response time.
Solution Approach 2:
The gates of high voltage transistors Q4 and Q5 are merged into a common gate node. This merging creates a unified control point that ensures both transistors respond identically to transient conditions, preventing the alternating on/off switching that would occur with separate gate controls and maintaining the blocking state during high frequency oscillations.
Data Source
AI summary
When a series protective element such as a transient blocking unit (TBU) is used in combination with a shunt protective element such as a gas discharge tube (GDT), firing of the GDT can cause a transient having the potential to damage the TBU. This problem can be alleviated by placing a TBU core in series between depletion mode transistors having their gates connected. With this arrangement, the GDT transient causes a transient in the TBU circuit that has the effect of switching the transistors around the TBU hard off, thereby protecting the TBU from the GDT transient.